Hydra-Matic History: GM’s First Automatic Transmission

GM’s original Hydra-Matic transmission was one of the most important innovations in the history of the automobile. It wasn’t the first automatic transmission, but it was the first one that really worked and its resounding commercial success paved the for every subsequent auto-shifter. This week, we take a look at the origins of the Hydra-Matic and its originator, Earl Thompson, who also developed the first Synchro-Mesh gearbox back in the 1920s.
Hydra-Matic hood badge on a 1942 Oldsmobile B-44 club coupe © 2009 Aaron Severson


In 2010, Ferrari raised the hackles of automotive purists with the announcement that it would shortly phase out its conventional manual transmissions in favor of F1-style sequential gearboxes. The announcement gave new fuel to an old debate: whether a conventional manual transmission and separate clutch pedal are fundamentally obsolete.

Outside of a small contingent of enthusiasts and professional drivers, the automotive world has long regarded the manual gearbox as at best a necessary evil. The multi-speed transmission, which dates back to the 1890s, evolved to compensate for the limitations of early engines, which had modest power and narrow rev bands. Getting a heavy car moving from rest required short (high numerical) gear ratios that would have the engine thrashing its guts out above about 15 mph (25 km/h), but a ratio optimized for brisk cruising — say, 40 mph (65 km/h)– wouldn’t have the torque multiplication to deal with steep hills. One of the great attractions of early electric cars, despite their severely limited range, was that they seldom required any gear changes at all, since electric motors produce their maximum torque from 0 rpm. Had early automakers devised a more efficient means of storing electricity, the evolution of passenger-car powertrains might have been very different indeed.

Actually shifting an early sliding-gear transmission was seldom a pleasant experience. Even upshifts often required careful timing and patience to avoid grinding gears while downshifts required double-clutching and rev-matching. Few people ever mastered those techniques, particularly since engine speed generally had to be judged by ear; tachometers were not common on mundane automobiles in those days. Then as now, there were a few who prided themselves on being adept with the gearbox, but they were definitely in the minority. Many drivers opted instead to shift to high as quickly as possible and then stay there for as long as possible.

This hatred of gear-shifting was not limited to the public. Henry Ford strongly preferred planetary (epicyclic) transmissions, so much so that he allegedly did not even learn to use a conventional gearbox until the development of the Model A in the mid-1920s. The Ford Model T’s pedal-operated planetary gearbox at least avoided clashing gears, although in practice it was scarcely less labor-intensive to use than a conventional gearbox.

Shortly after the Great War, a number of engineers, including Britain’s Walter Gordon Wilson and France’s Jean Cotal, developed more sophisticated “preselector” planetary transmissions. With a preselector gearbox, you chose a ratio with a selector lever and then engaged that gear by pushing the gear selector pedal, which took the place of the traditional clutch. Preselector gearboxes were easier to use than was a conventional manual transmission, but they were also less efficient, substantially costlier, and often too complex to be completely trustworthy. As a result, they never quite caught on for passenger car use, although Cotal and Wilson preselectors were used in a number of pricier British and French cars (and quite a few British buses) into the fifties.

1937 Cord 812SC Cabriolet dashboard © 2012 David Berry (CC BY 2.0 Generic)
The short-lived, front-wheel-drive Cord 810/812 was one of the few American production cars to use a preselector gearbox — a four-speed unit with Bendix “Electric Hand” controls. The electromagnetically controlled Electric Hand system, also available on some Hudsons (with a standard three-speed gearbox), was adopted because it simplified the linkage between the shifter and the front-mounted gearbox, but it was quite troublesome in service. (Photo: “1937 Cord 812 SC Cabriolet” © 2012 David Berry; used under a Creative Commons Attribution 2.0 Generic license)

The preselector transmissions were automatic in a sense, but, with a number of rare exceptions beyond the scope of this article, they did not relieve the driver of the need to select the appropriate gear ratio for any given circumstance. Although there had been experiments with autonomously self-shifting transmissions since at least 1904, the technical challenges remained substantial and none of the various attempts had been reliable enough or practical enough to have much success. A conventional dual-shaft transmission, whatever its deficiencies, was at least a known quantity.


Among the many people searching for easier ways to change gears was a young hydraulics engineer from Oregon named Earl A. Thompson. In 1918, Thompson applied for a patent on a three-speed preselector transmission that used a drum-shaped synchronizer to match the speed of each newly selected gear with the speed of the transmission output shaft prior to engaging that gear. The idea was that the driver would select the desired gear and then press the clutch pedal, which would disengage the clutch, synchronize the speeds of the selected gear and the output shaft, complete the shift, and then reengage the clutch automatically.

Thompson continued to develop and refine this idea, filing a second patent disclosure in 1923 that included a new synchronizer mechanism using cone clutches to match the speeds of gears to be meshed. He also managed to build a functional prototype of his preselector transmission, which he installed in a new Cadillac donated by his younger brother Kirk, a Portland-area Cadillac dealer.

That September, Thompson and his brother drove the car to Detroit, where Thompson hoped to sell his invention to the auto industry. Although Detroit’s usual reaction to outside inventions bordered on the categorically hostile, the Thompson brothers managed to secure an audience with Cadillac chief engineer Ernest W. Seaholm and then entered preliminary discussions with GM’s New Devices Committee about the possibility of GM’s purchasing or licensing Thompson’s patents. Those negotiations went nowhere, but Seaholm, who had found Thompson’s design crude but interesting, convinced Cadillac general manager Herbert Rice that Cadillac itself should take on the development of Thompson’s invention.

Thompson resettled in Michigan and went to work as a Cadillac consultant, developing his original ideas into production form. His preselector transmission concept was discarded — judging by the patent description, it added a lot of complexity to no obvious benefit — but Cadillac remained very interested in his gear synchronizer mechanism, which would work just as well in an otherwise conventional transmission. After exhaustive testing and more than two dozen prototypes, Cadillac finally put Thompson’s invention into production in August 1928. The new transmission, dubbed “Silent Synchro-Mesh,” debuted that fall on the 1929 Cadillac and LaSalle.

For cost reasons, the early Synchro-Mesh transmissions provided synchronization only between second and third gears, so shifting into first still generally required coming to a complete stop to avoid clashing. (The “all-synchro” transmission with synchronized low gear wouldn’t become universal until around 40 years later.) Still, the system was a considerable improvement on earlier ‘crashbox’ transmissions and made driving a good deal less painful. Synchro-Mesh quickly spread to other GM divisions and was subsequently licensed or imitated by many other automakers in the U.S. and Europe. By the mid-thirties, most passenger cars had some form of synchronized transmission. (After the war, Thompson’s gear synchronization design would eventually have a strong rival in Porsche’s patented balk ring system, but that’s another story.)

Thompson became a Cadillac employee roughly a year after the first Synchro-Mesh cars debuted. About a year after that, Seaholm and general manager Lawrence P. Fisher (who had succeeded Rice in May 1925) promoted Thompson to assistant chief engineer. He had taken much of the teeth-gnashing (both literal and figurative) out of shifting. The next step was to make the process automatic.

1929 Cadillac 841-B convertible coupe front 3q © 2006 www.RemarkableCars.com, released for all use with proper attribution
Both Cadillac and its cheaper companion make, LaSalle, gained Silent Synchro-Mesh for the 1929 model year. This 1929 Cadillac 841-B is powered by a 341 cu. in. (5,578 cc) V-8 with about 90 gross horsepower (67 kW); we believe this is a Fisher-bodied convertible coupe. (Photo: “1929-cadillac-archives” © 2006 www.RemarkableCars.com (photographed 2005 by Douglas Wilkinson at en.wikipedia); released for all use (with proper attribution) by the copyright holder, resized 2010 by Aaron Severson)


When talking about the origins of GM’s early automatic transmissions, it’s important to understand that for the first seven decades of its history, the corporation was not nearly as monolithic as the modern enthusiast or historian might assume. Each division operated more or less independently and was largely autonomous, responsible for its own engineering, manufacturing, and sales. There were occasional collaborative projects, but in general, if the divisions needed something engineered or manufactured by another GM division, they had to buy it like any other customer.

While each division did much of its own R&D work in those days, GM also had central Research Laboratories, headed from 1920 to 1947 by the inimitable Charles F. Kettering, famously the inventor of the automotive self-starter. The research engineers operated independently of the production divisions, conducting advanced engineering and research projects to develop technology (not necessarily automobile-related) that could be adopted by different GM divisions and/or licensed to outside companies. The Research Laboratories worked on all manner of projects, ranging from high-compression engines and leaded gasoline to hydraulic valve lifters.

In the twenties, the work of the Research Laboratories was primarily on the theoretical and experimental side. If a particular invention seemed worthwhile, one or more divisions would be drafted (not always happily) to work with the research engineers to develop the idea for production. In 1931, the corporation organized a central Engineering Staff, led by VP of engineering Ormond E. Hunt, that could serve as a bridge between the research engineers and the divisions. However, ultimate responsibility for the production version of any specific concept or invention still (usually) rested with the individual division, which sometimes led to different divisions offering several distinct variations on the same basic technological theme.

As you would expect, the Research Laboratories worked throughout the twenties to find alternatives to the dual-shaft transmission, exploring a wide variety of electromagnetic, hydraulic, and friction drive systems. These efforts took on some additional urgency after Alfred P. Sloan became president of General Motors in 1923. While he was no fan of engineering novelty for its own sake and had strongly opposed some of Kettering’s wilder ideas — such as the ill-fated “Copper Cooled” Chevrolet — Sloan was by his own admission a mediocre driver who could not use a conventional gearbox with any skill. Recognizing that there were many like him, Sloan understood that a reliable and effective self-shifting transmission would have powerful commercial potential.

Cadillac became involved in this work in the late twenties or early thirties when the division was assigned to support Buick in the development of an ambitious infinitely variable friction drive transmission that the Research Laboratories’ Dynamics unit had conceived. (It appears the friction drive unit was based on one or more outside patents that GM had either purchased or licensed, although the scant information we’ve found on the design’s origins is confusing and contradictory.) Nicknamed the “Roller,” the transmission used two sets of toroidal races, one set driven by the engine, the other set connected to the output shaft; power was transmitted between the races by a series of adjustable rollers.

The Roller’s development was protracted and difficult. The friction drive transmission was extremely smooth and potentially very efficient, but its mechanical complexity made it frighteningly expensive and its reliability remained at best suspect. Cadillac eventually withdrew from the development in favor of an entirely different project, conceived in-house by Thompson. It was just as well; Buick would never offer a production version of the Roller.

Thompson’s ideas on automatic transmission focused not on friction drive, but on planetary gearsets. Cadillac had actually used epicyclic transmissions many years earlier, although that had been well before Thompson’s time; Cadillac switched to dual-shaft transmissions back in 1908. Thompson’s direct inspiration was the 1931 Daimler Double Six, a copy of which Seaholm had purchased for evaluation in late 1930 or early 1931. The Daimler was fitted with the four-speed Wilson preselector transmission and a novel new feature: the Fluid Flywheel, the first fluid coupling ever offered in a production passenger car. (See the sidebar below.)

Even with the Fluid Flywheel, the Wilson gearbox still required manual gear selection, but Thompson recognized that the combination contained most of the ingredients for a practical fully automatic transmission.

SIDEBAR: Fluid Couplings

Although the British automaker Daimler was the first to offer a fluid coupling on a production automobile, the fluid coupling (or fluid clutch) was by then more than 20 years old. It was first patented back in 1905 by Hermann Föttinger, a Bavarian engineer then working for the shipbuilder Stettiner Maschinenbau AG Vulcan, and became fairly common in marine applications after World War I. In the late twenties, the British engineer Harold Sinclair adapted the Vulcan fluid coupling for industrial and bus applications. It was the latter version, with some additional refinements by Daimler chief engineer Laurence Pomeroy, that became the Daimler “Fluid Flywheel” in 1930.

As the vast number of related patents will attest, fluid clutches can have many possible variations, but at its simplest, a fluid coupling consists of two ring tori — torus-shaped discs — facing each other in a closed housing filled with fluid, usually some type of light oil. The inner surface of each torus member is covered with radial vanes that function much like the blades of a fan or a propeller. One torus member, called the impeller or pump, is driven by the engine. As the impeller spins, its vanes transmit that motion through the oil to the other torus member (called the turbine), which is geared or splined to an output shaft. The oil then returns to the impeller to begin the cycle over again. Once the force applied to the turbine blades by the moving oil is strong enough to overcome the turbine’s inertia (a point known as stall), the turbine and its output shaft begin to rotate.

Color diagram of a simple two-element automotive fluid coupling © 2016 Aaron Severson
A much-simplified diagram of a typical automotive fluid coupling (not, we should emphasize, that of Hydra-Matic!). The engine (not shown) drives the flywheel or flex plate; the flywheel drives the impeller through the front torus cover (dark red). The impeller creates motion in the oil within the torus housing, which turns the turbine; the turbine drives the output shaft (blue). (author diagram)

Unlike a mechanical clutch, a fluid coupling never transfers 100% of the motion of the flywheel; some of the input torque is always lost to friction heat (slip) within the moving fluid, just as a slipping plate clutch loses energy to friction between the clutch surface and the flywheel. Fluid couplings are particularly inefficient at low speeds because any time the impeller is turning significantly faster than the turbine, oil actually leaves the turbine with a rotational velocity opposite that of the impeller. Slippage decreases as the turbine accelerates, allowing the coupling to reach an efficiency of 97% or better at cruising speed (when turbine and impeller speeds are nearly equal), but the only way the coupling will ever be 100% efficient is if the two torus members are somehow bypassed or mechanically locked together.

(The torque converter, also invented by Föttinger and described in his 1905 German patent disclosure (albeit not under that name), is a fluid coupling with an additional member: a bladed disc called a stator, mounted on a fixed shaft. At low speeds, fluid leaving the turbine hits the blades of the stator, reversing the fluid’s direction before the fluid reenters the impeller. That change of direction increases the efficiency of the impeller so that at certain speeds the impeller multiplies the engine’s torque rather than simply transmitting it. Unlike a mechanical gear ratio, however, the torque multiplication quickly fades as the impeller and turbine speeds increase.)

A fluid coupling or torque converter’s low-speed slippage provides two important advantages for passenger cars: First, it provides the slip necessary to start the car from rest. Second, a car with a fluid clutch will not stall the engine if the car comes to a stop while in gear as would be the case with a conventional plate clutch. Combined with a planetary gearbox, like GM’s Hydra-Matic, a fluid coupling can completely eliminate the need for a conventional clutch pedal. (Nevertheless, some semiautomatic transmissions, like Chrysler’s Fluid Drive, had both a friction clutch and a fluid coupling; manual de-clutching was necessary for stopping and starting.)

The main drawback of a fluid coupling is that its slippage wastes power, potentially hurting both acceleration and fuel economy. Some bus and coach transmissions (and the postwar Packard Ultramatic and Borg-Warner DG) solved that problem by using a mechanical clutch to lock the flywheel to the transmission input shaft at cruising speeds, a feature that became virtually universal for torque converter automatics in the late 1970s. Hydra-Matic took a different approach, which we’ll discuss later in this article.


In early 1932, Seaholm assigned engineers Ralph F. Beck and Walter B. Herndon to assist Thompson with his automatic transmission project, which was dubbed the “Military Transmission.” Although the project would eventually have military applications, the moniker was just a codename, intended — like the ominous “Keep Out” sign hung outside the door — to discourage prying eyes.

The Military Transmission project’s objective was to develop a planetary gearset that could be operated automatically by means of hydraulic servos. The first fruit of this work, on which Thompson filed a patent in March 1933 (subsequently U.S. Patent No. 2,285,760), was a conventional sliding-gear transmission augmented by a two-speed planetary gearset that provided automatic shifting between direct drive and overdrive ratios. The design was similar in broad principle to the new Reo Self-Shifter, patented two years earlier and announced in May 1933 as a production option for the 1934 Reo Royale and S-4 Flying Cloud. For Thompson, the semiautomatic transmission appears to have been mostly an early essay in speed-sensitive hydraulic governor systems, suggesting the direction of his thinking.

Around the time Thompson’s patent was filed, Seaholm expanded Thompson’s group from three engineers to five, adding William L. Carnegie and Maurice S. Rosenberger to the team. They soon began work on a more elaborate hydraulically operated planetary transmission, which had reached the prototype stage by mid-1934. Thompson applied for a patent on it that October.

Unfortunately, the entire project was rapidly becoming an unaffordable expense. For several years, Cadillac general manager Larry Fisher had been spending lavishly on new products and new technology, including double wishbone suspension, power steering (which for various reasons Cadillac wouldn’t actually offer until 1952), and of course the V-16 and V-12 engines. The results were often impressive, but with Cadillac sales slumping badly as the Depression worsened, the division’s future was very much in question. Service manager Nicholas Dreystadt, who succeeded Fisher as general manager in June 1934, convinced the corporation to give Cadillac a reprieve, but severe budget cuts left the Military Transmission project hanging by a thread.

Epicyclic gear diagram © 2005 Wapcaplet (CC BY-SA 3.0 Unported)
A 3-D diagram of a planetary (a.k.a. epicyclic) gearset. Each planetary gearset can provide different ratios depending on which elements are driven and which are locked in place. A single planetary gearset can provide reduction gearing, direct drive, or overdrive in either forward or reverse, although not all gearsets are capable of all these combinations. Planetary gearsets can also be used as torque-splitting or torque-combining differentials, dividing output torque in multiple directions or applying the combined torque of several different engines or prime movers to a single driveshaft. (Illustration: “Epicyclic gear small” © 2005 Wapcaplet; used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

By this time, Buick’s friction drive transmission had been canceled, so for Thompson’s work to go the same way would have been a significant setback to an effort that still had Sloan’s strong personal interest. The eventual answer was to transfer Thompson’s project to O.E. Hunt’s corporate Engineering Staff, which had its own facilities and budget. In January 1935, Thompson and his team moved to the central offices to become a corporate product study group, later named the Transmission Development Group.


By this time, Thompson had developed a new four-speed semiautomatic transmissions, patent applications for which were filed in October 1934 and October 1935 (U.S. Patent Nos. 2,195,605 and 2,193,304 respectively). The transmission used two servo-operated planetary gearsets with an additional set of spur gears, interposed between the clutch and front gearset, for neutral and reverse. Thompson specified a conventional single-plate friction clutch, but it was strictly a stopgap. He still wanted to eventually substitute a fluid coupling, but his tiny team hadn’t yet had time to design a suitable one.

The semiautomatic transmission’s rear servo (and thus the rear gearset) was controlled by the driver using a selector lever on the steering column, but the front servo functioned automatically, shifting from direct drive to reduction and back based on throttle position and road speed; the latter was signaled by a centrifugal governor driven by the transmission output shaft.

The idea was that the driver would start in Neutral, disengage the friction clutch with the clutch pedal, and select Low for a first-gear start, followed in short order by an automatic shift to second. Obtaining third and fourth required shifting manually from Low to High (which did not require de-clutching). The transmission would then shift automatically between third and fourth gears.

Some time after Thompson’s group moved to the Engineering Staff, their project came to the attention of Oldsmobile general manager Charles L. McCuen. We don’t know if McCuen was aware of Buick’s now-canceled friction drive project, but he was in any case very keen for Oldsmobile to have an automatic transmission of its own. He ordered Oldsmobile chief engineer Harold T. Youngren to work closely with Thompson’s group to adapt their ideas into something Oldsmobile could build and sell.

Later that year, senior corporate management decided that the manufacturing portion of the equation should be handled by Buick, which had unused factory space that could be retooled for the purpose. Buick was also ordered to share the new transmission, presumably as a sort of consolation prize for the abortive Roller project. That directive sat ill in Flint; Buick engineers still had their own ideas about automatic transmission (some of which would be realized in the postwar Dynaflow) and had no love for the Automatic Safety Transmission, which they hadn’t developed and didn’t want.

Throughout this period, Thompson continued to refine the semiautomatic transmission, applying for an additional patent in March 1937 (U.S. Patent No. 2,362,418). Production began around the same time and the new transmission, which Oldsmobile dubbed the “Automatic Safety Transmission” (AST), went on sale in June as an option for the 1937 Oldsmobile Eight. List price was initially $80, rising to a hefty $100 that fall, when availability was extended to six-cylinder Oldsmobiles and the Buick Series 40 Special. Buick doesn’t appear to have bothered coming up with a trademark for the semiautomatic, describing it simply as a self-shifting transmission, but it was otherwise identical to the Oldsmobile unit.

SIDEBAR: Automatic Safety Transmission Operation

Although the Automatic Safety Transmission is usually called a four-speed semiautomatic transmission, Earl Thompson’s patent disclosures actually describe it as essentially three separate transmissions in series: a synchronized dual-shaft reverse unit with helical gears followed by two separate two-speed planetary transmissions.

In a car equipped with the Automatic Safety Transmission, the engine drove a conventional single-plate clutch, whose driven shaft drove the input shaft of the reverse unit. The reverse unit provided the driver with three choices: neutral, reverse reduction, or forward direct drive.

With the reverse unit in gear, that unit’s output shaft drove the annulus (ring gear) of the front planetary transmission, which was surrounded by a brake drum connected to the planetary gearset’s central sun gear. A band brake could be engaged to lock the drum and thus the sun gear in place, putting the planetary unit in reduction. The drum also carried the driving plates of a multi-disc clutch pack, which could be engaged to lock the drum and the sun gear to the planetary unit’s planet carrier.

The front unit planet carrier drove the dual sun gears of the rear planetary transmission, whose first annulus was connected to another brake drum and multi-disc clutch pack that could either hold the first annulus in place or else lock both the first planet carrier and both ring gears together to put the unit in direct drive. The second planet carrier drove the output shaft.

Both planetary gearsets were controlled by a combination of spring and hydraulic pressure. A servo could be engaged to release the brake band while a control valve could apply hydraulic pressure to engage the clutch pack. The difference was that the rear unit was controlled manually by the driver moving the selector lever while the front unit was controlled automatically. These two gearsets could be compounded as shown in the table below. (In this table, “ENG” means “ENGAGED” while “REL” means “RELEASED.”)

Automatic Safety Transmission Gearing Sequence
Front Planetary Gearset (Automatic) Rear Planetary Gearset (Manual)
Gear Servo Band Clutch Ratio Servo Band Clutch Ratio Overall Ratio
* Although the front servo was released in neutral, the front band normally did not engage because no power was applied to the input shaft and front annulus.
Neutral REL OFF REL 0* ENG OFF ENG 1.00
1st REL ON REL 1.42 REL ON REL 2.23 3.17
2nd ENG OFF ENG 1.00 REL ON REL 2.23 2.23
3rd REL ON REL 1.42 ENG ON ENG 1.00 1.42
4th ENG OFF ENG 1.00 ENG OFF ENG 1.00 1.00

The Automatic Safety Transmission did not actually give the driver direct control of gear selection except in Reverse and Neutral. Instead, the selector provided Low and High ranges, which provided provided automatic shifting between first and second and third and fourth gears respectively. Automatic upshifts were triggered mostly by road speed, which was signaled by a centrifugal governor driven by the output shaft. However, a throttle-controlled mechanical linkage could either delay the upshift or force a downshift within certain speed ranges.

Oldsmobile cautiously recommended starting in Low, but, as Thompson’s patent disclosures noted, starting from rest in High would actually cause the transmission to start in first gear and then shift automatically into third. Buick sales literature presented this as a feature, but it was really more of a known bug, resulting from a quirk of the hydraulic system layout. Idling in Neutral with the clutch engaged pressurized the rear servo, putting the rear planetary in direct drive (as in High range); this was intentional to facilitate push-starting. Disengaging the clutch, however, cut off power to the transmission oil pump (again by design) while moving the shift lever to the Low position dumped pressure from the rear servo. If you then started in High, it took a moment or two for the pump to build up enough hydraulic pressure to reengage the rear servo. Once it did, the rear planetary gearset would then shift from reduction to direct drive and belatedly put the transmission in third gear.

As a result, owners seldom bothered using Low, whether the owner’s manual recommended it or not. First, third, and fourth were perfectly adequate for most conditions and not having to shift from Low to High was obviously more convenient, although the practice did the transmission’s reliability no favors.

Oldsmobile advertising claimed that cars with the Automatic Safety Transmission returned up to 20% better fuel economy than did cars with the standard three-speed manual transmission. That claim, which in practice was rather optimistic (Buick claimed only 8%), was not due to the efficiency of the semiautomatic transmission, which consumed more power than did a conventional gearbox, but rather to the use of a significantly taller (lower numerical) axle ratio than was specified with the standard three-speed.

By the time the Automatic Safety Transmission appeared on the market, Reo’s Self-Shifter had come and gone and buyer interest in semiautomatic transmissions had proven to be limited. Not only were the transmissions complicated, expensive, and often troublesome, they were still not really automatic. While their operation was different than that of a conventional transmission, saying they were easier to use (much less safer, as Oldsmobile claimed) was arguable, particularly since they retained the clutch pedal, of which many American motorists would have been happily rid. (This lesson was apparently lost on Chrysler, which by 1937 was working on its own M3 semiautomatic transmission for a 1940 introduction.)

It certainly didn’t help that the Automatic Safety Transmission wasn’t very reliable, particularly early on. The transmission’s teething problems were extensive and Oldsmobile found that Buick engineers and production people, eager to wash their hands of the project, were not much help. The transmission was also unfamiliar territory for technicians, and Oldsmobile did not encourage tinkering, in part because it took a while for even the factory production engineers to figure out how to resolve certain common issues. As a result, dealers responded to most problems by pulling the transmission and replacing it with a new factory-refurbished unit. (Substituting a standard gearbox, which some unhappy owners would probably have preferred, was a more complicated chore that required also replacing the steering column, the entire driveshaft, and ideally the rear axle gears.)

1938 Oldsmobile Automatic Safety Transmission advertisement
An Oldsmobile ad for the 1938 Automatic Safety Transmission. The transmission’s supposed safety advantage was that it reduced the need for the driver to remove one hand from the steering wheel to change gears. (ad image: “1938 Oldsmobile Eight & Six Sedans” courtesy Alden Jewell; used with permission)

Buick dropped the semiautomatic transmission after only a year and never offered the unit in their bigger cars, but Oldsmobile persisted through the 1939 model year, reducing the list price by $25 for the transmission’s final season. Total production, which continued through September 1939, was limited. We have no exact figures, but we’ve seen estimates ranging from about 15,000 to as many as 40,000. Buick took only about 3,000 of those for itself, so most went into 1937–1939 Oldsmobiles. Warranty costs were high and the Automatic Safety Transmission’s suggested retail price was considerably less than Oldsmobile paid Buick for each transmission, so Oldsmobile undoubtedly lost money on the whole endeavor. However, if nothing else, the project provided plenty of real-world experience for the fully automatic transmission Thompson and McCuen still hoped to offer.

SIDEBAR: Oscar Banker’s Automatic

Even before Earl Thompson’s team started work on its semiautomatic and automatic transmissions, Armenian Oscar H. Banker (born Asadoor Sarafian) was developing his own automatic transmission concepts, applying for a host of patents in that area between 1927 and 1937.

Banker’s transmission-related patents covered an assortment of two-, three-, and four-speed designs, some using spur gears, some using planetary gearsets, and some with a combination of the two. Unlike Thompson’s designs (or later Oliver K. Kelly’s), Banker’s automatic transmissions were controlled mechanically by one or more centrifugal governors rather than by hydraulic servos. Also, Banker mostly eschewed brake bands in favor of one-way clutches, a feature GM would adopt for the second-generation Controlled Coupling Hydra-Matic about 20 years later.

According to Banker, he demonstrated a prototype of his transmission in late 1930 to O.E. Hunt, Larry Fisher, Ernest Seaholm, Thompson, and Charles Kettering. Banker claimed that GM’s New Devices Committee was prepared to license the design, but Banker’s business partner and financial backer, Franklin Magill of the Magill-Weinsheimer Company, wanted more money than GM was willing to pay. In 1934–1935, however, GM’s Yellow Coach subsidiary (the Yellow Truck and Coach Manufacturing Company) did briefly license one of Banker’s transmission designs, which was used under the name Monodrive (sometimes styled “Mono-Drive”) in a modest number of buses for the Chicago Motor Coach Company. Magill later secured a separate license agreement with Borg-Warner.

The author is not an engineer or a lawyer, much less a patent lawyer, so we’re not qualified to weigh the legal merits of Banker’s later allegation that Hydra-Matic infringed his patents. While we don’t doubt that Thompson’s group was aware of Banker’s work and probably would have been even if Thompson and Banker had never actually met (it’s both customary and prudent to pay close attention to rival inventors’ published work), consciously developing a competing design does not necessarily constitute an infringement. The question of whether any specific elements of Thompson and Kelley’s transmission designs infringed Banker’s patents would have had to be weighed by a federal court. Since to our knowledge neither Banker nor his licensees ever pursued formal legal action, there seems little point in speculating on the potential outcome of such a case.

That said, having examined many (though not all) of the patents in question, our layman’s observation is that Thompson and Kelley’s designs appear quite a bit different from Banker’s in both principle and operation. Furthermore, many of the broad similarities that do exist, such as the use of planetary gearsets, were already established in prior art. Neither Thompson nor Banker invented the epicyclic gear train, nor for that matter the idea of an automatic transmission — brothers Thomas L. and Thomas J. Sturtevant patented the first version of their centrifugally operated automatic transmission in 1904, when Thompson and Banker were still children.

A patent search on Banker’s name reveals more than a dozen patents related to automatic transmissions, but if you’re curious, the most pertinent are probably U.S. Patent Nos. 1,795,464 (assigned to Continental Illinois Bank and Trust Company, filed 21 October 1927, issued 10 March 1931); 1,795,465 (filed 26 November 1928, issued 10 March 1931); 1,843,193 (assigned to Continental Illinois Bank and Trust Company, filed 9 April 1930, issued 2 February 1932); and 2,262,747 (assigned to New Products Corporation, filed 18 September 1936, issued 31 January 1942, and reissued 18 May 1943).


Even before the Automatic Safety Transmission went on sale, Thompson’s group was already working to make the semiautomatic transmission obsolete. All that remained was to eliminate the clutch pedal and design a hydraulic control unit smart enough to autonomously manage all shifting in normal driving.

Thompson’s solution to the latter problem was outlined in the patent disclosure he filed in April 1938 (U.S. Patent No. 2,204,872). It again described a four-speed transmission using two planetary gearsets and an additional set of spur gears for neutral and reverse. As in the Automatic Safety Transmission, the planetary gearsets’ brake bands and clutches were operated by servos. Oil flow to each servo was controlled by a separate hydraulically operated valve, each of which was now operated by a spring-loaded control piston.

Actuating pressure for those pistons was supplied by an engine-driven centrifugal governor whose output (in this iteration of the invention) was proportional to engine speed. However, the earlier designs’ completely mechanical throttle linkage was deleted in favor of a new throttle-controlled hydraulic compensator line that served to vary the amount of hydraulic pressure necessary to operate the control valves. The wider the throttle was opened, the more governor pressure it took to move each piston rod and open the associated shift valve. In this way, each forward gear ratio was ‘mapped’ to a specific range of governor output pressure. Pressure exceeding or falling below that range would trigger an automatic shift up or down, respectively.

In essence, the hydraulic controls now functioned as an analog computer that could be programmed to shift autonomously between all available forward gears at different combinations of engine speed and throttle setting. The throttle valve gave the driver some ability to either hasten or delay gear changes, but not to override them. Therefore, you couldn’t hold a lower gear past redline, deliberately lug the engine in high gear under load, or force a downshift that would over-rev the engine. (As in the earlier semiautomatic transmission, there was still a separate Low range which could be manually selected to lock the control valve for the rear unit servo in the closed position, which limited automatic shifts to first and second gears.)

The table below summarizes the actual gearing sequence and ratios for the new transmission when it made its production debut in late 1939. (As in the previous table, “REL” means “RELEASED” and “ENG” means “ENGAGED.”)

Prewar Oldsmobile Hydra-Matic Gearing Sequence
Front Planetary Rear Planetary Reverse Planetary
Gear Band Clutch Ratio Band Clutch Ratio Pawl Ratio Overall Ratio
* In Neutral, the rear band is applied with the engine off, but released with the engine running.
† Negative signifies reverse.
1st ON REL 1.44 ON REL 2.53 OFF 3.66
2nd OFF ENG 1.00 ON REL 2.53 OFF 2.53
3rd ON REL 1.44 OFF ENG 1.00 OFF 1.44
4th OFF ENG 1.00 OFF ENG 1.00 OFF 1.00
Reverse ON REL 1.44 OFF REL ON -2.99† -4.31†

The new transmission’s final major element — a fluid coupling that could replace the Automatic Safety Transmission’s plate clutch — was outlined in the patent filed in February 1937 (U.S. Patent No. 2,176,138) by Oliver K. (“O.K.”) Kelley, who had worked with Thompson at Cadillac and later joined him in the Transmission Development Group after a stint at GM’s Yellow Coach and Truck subsidiary.

Although it was actually patented more than a year before Thompson’s hydraulic controls, it seems more appropriate to discuss Kelley’s design second. While Thompson’s patent was essentially an extrapolation of the group’s previous semiautomatic transmissions, Kelley’s was a substantially new design with some significant variations. Not only did it use a fluid coupling rather than a plate clutch, there were now three planetary gearsets rather than two (although there were still only four forward speeds) and no spur gears; the third gearset was now used to provide reverse reduction. There was also an important new feature: an additional intermediate shaft, concentric with the transmission main shaft, whose leading end was permanently attached to the impeller. The intermediate shaft, which was driven by the planet carrier of the front planetary gearset, served to connect the impeller to the clutch assembly of the second planetary gearset.

The intermediate shaft served two important functions. First, it provided an indirect connection between the engine and the impeller of the fluid coupling. With most fluid clutches, the impeller is bolted or otherwise attached directly to the engine flywheel — in automotive applications, typically through the coupling’s torus housing. In Kelley’s design, the engine instead drove the annulus (ring gear) of the front planetary gearset. The annulus then drove that gearset’s planet pinions and planet carrier, which in turn drove the immediate shaft and with it the impeller. As a result, the impeller and the engine only turned at the same speed if the front clutch were engaged, which locked the front planetary gearset in direct drive (which in this case happened in second or fourth). With the front clutch released and the front brake band engaged (i.e., in first, third, or reverse), the speed of the impeller would be reduced by the ratio of the front gearset. The purpose of this unusual arrangement was to deliberately reduce the efficiency of the coupling at idle and off-idle speeds to provide smoother takeoffs and minimize ‘creep’ in first or reverse without hampering efficiency at higher speeds.

The intermediate shaft’s second purpose was to reduce slippage in the cruising gears — third and fourth — by providing a partial mechanical connection between the engine and the second planetary gearset in those gears. As we mentioned above, the intermediate shaft also drove the second unit clutch hub. Engaging the second unit clutch (as in third or fourth) caused the intermediate shaft to simultaneously drive both the fluid coupling impeller and the second unit annulus, creating a “split torque” arrangement. Some of the torque on the intermediate shaft was still applied to the impeller and thus the fluid coupling impeller, turbine, main shaft, and second unit sun gear, but most — more than 60% — was now applied directly to the second unit annulus. (The actual torque split depended on how many teeth the annulus and sun gear each had; in the earliest production Hydra-Matic, the split was 60.5% mechanical, 39.5% hydraulic.)

The second unit planet carrier then reintegrated these two torque inputs in the same manner as a marine or locomotive transmission using multiple engines to turn a common shaft. This effectively reduced coupling slip by more than 60% in both third and fourth gears. (It didn’t actually prevent the coupling from slipping, but it allowed a substantial portion of intermediate shaft torque to bypass the coupling. For a further explanation of this principle, see our article on split torque transmissions.)

1940 Oldsmobile Series 70 four-door sedan front 3q © 2014 Sicnag (CC BY 2.0 Generic)
Hydra-Matic was optional even on the cheapest six-cylinder Oldsmobile Series 60 in 1940, although we suspect it was more common on the larger and more expensive Series 70 and Series 90 models. The Series 60 and Series 70 both shared Oldsmobile’s 95 hp (71 kW) 230 cu. in. (3,536 cc) inline six, but the Series 70 had a taller 4.30 axle (3.63 with Hydra-Matic), compared to the Series 60’s 4.10 (3.42 with Hydra-Matic), to compensate for the extra weight of the larger body; a Series 70 four-door sedan weighed 113 lb (51 kg) more than the equivalent Series 60. (Photo: “1940 Oldsmobile Dynamic Series 70 Sedan” © 2014 Sicnag; used under a Creative Commons Attribution 2.0 Generic license)

Even Kelley recognized that this arrangement was more complicated than it probably needed to be, but the underlying principles were solid and by this time mostly well-understood. If the design was inelegant, it was at least functional and, just as important, production-feasible.


Production feasibility was still a major priority for Oldsmobile, which remained actively involved in the development process. Based on the timetable, it appears that as soon as Thompson’s team came up with a viable-seeming idea, it was handed off to Oldsmobile engineers for evaluation and testing; McCuen wanted something that could replace the Automatic Safety Transmission in the near future. By early 1939, Oldsmobile engineer Harold N. Metzel was already overseeing the road-testing of some 5,000 preproduction examples of the new transmission.

Inevitably, the transmission underwent further changes before reaching production; some but not all of those changes are described in Kelley’s April 1939 patent disclosure (subsequently U.S. Patent No. 2,211,233). Thompson’s single oil pump, which as originally described was driven both by the engine and the tail shaft, was replaced with separate front and rear pumps, the shaft of the latter also driving the governor assembly. As a result, shift points of the production transmission were once again governed primarily by road speed rather than engine speed. The design and layout of the shift valves and throttle-controlled pressure lines was extensively revised (although the basic principle was still as described above). Finally, Thompson developed a new and significantly more efficient fluid coupling design, patent applications for which were filed in 1940 (U.S. Patents 2,357,295 and 2,430,258).

The other major mechanical change was a completely redesigned third (reverse) planetary gearset. In reverse, the second and third units acted together as a single compound gearset. The second-unit annulus was attached to and rotated with the third-unit sun gear while both the second- and third-unit planet carriers were splined to the output shaft. Reverse rotation was provided by releasing both the clutch and the brake band of the second unit. Rotation of the second-unit sun gear, still driven by the fluid coupling turbine through the main shaft, would then cause the second-unit annulus and third-unit sun gear to rotate backward. Engaging a toothed reverse pawl locked the third-unit annulus in place, turning the carriers and thus the output shaft backward in reduction. The reverse pawl also served to lock the driveshaft with the engine off.

Color diagram of 1940–1947 Model 180 Oldsmobile Hydra-Matic
A diagrammatic representation, not to scale, of the early Hydra-Matic, omitting the servos and valve body (located below the planetary gearsets) and the multitude of hydraulic lines. Each color represents components that are integral or otherwise permanently attached so that they rotate together. As you can see by studying this diagram, the rear planetary gearset provides a torque-splitting function in third and fourth gears. In those gears, the rear annulus (light blue), driven by the intermediate shaft (red), turns at the speed of the impeller while the rear sun gear (medium blue), driven by the main shaft (also medium blue), turns at turbine speed. This rotates the planet carrier and the output shaft (purple) a little slower than the impeller, but still faster than the turbine, which is subject to hydraulic slippage. (Author diagram)

The resulting transmission still bore a clear resemblance to the Automatic Safety Transmission it would shortly supersede, but it no longer required a clutch pedal or much driver intervention in most normal operation. In High, the transmission would start in first and then shift for itself through all four gears. The previous Low range was retained, allowing the driver to keep the transmission in first and second at speeds up to about 40 mph (64 km/h), but that was usually only necessary for steep hills or perhaps hauling a heavy load at low speeds.

GM christened the new transmission Hydra-Matic. Although Oldsmobile would have exclusive use of it for the first year — a corporate policy acknowledging the considerable resources the division had poured into the transmission’s development — GM had big plans for the automatic. A new Detroit Transmission Division, headed by Victor A. Olsen, was established specifically to manufacture Hydra-Matic. William Carnegie, who had been with Thompson’s group since 1933, was appointed chief engineer of the new division, which got its own assembly facilities in a former Fisher Body plant in eastern Detroit.

1942 Oldsmobile B-44 Special Sixty Six club coupe Hydra-Matic shift quadrant © Aaron Severson
The shift pattern of prewar Hydra-Matics was N-Hi-Lo-R. Unlike the earlier Automatic Safety Transmission, High used all four forward speeds; postwar shift quadrants re-labeled this position “Drive,” which was more accurate and probably less confusing. (author photo)

Oldsmobile received the first production Hydra-Matic transmissions (known internally as Model 180) that October. When the 1940 Oldsmobiles debuted late that year, Hydra-Matic was optional across the line at a low introductory price of $57 — actually $19 less than the 1939 price of the Automatic Safety Transmission. As with the semiautomatic transmission, Hydra-Matic included much taller, economy-oriented 3.42 or 3.63 axle ratios (compared to 4.10 or 4.30 for manually shifted cars).

Oldsmobile advertising was predictably breathless about Hydra-Matic, extolling its ease of use and reduced fuel and oil consumption. If the latter claims were again optimistic (Oldsmobile started off claiming savings of up to 20%, soon amended to 10–15%), the other boasts were well-earned. Hydra-Matic was a genuinely paradigm-changing innovation that finally made good on the promises its predecessors hadn’t quite fulfilled.

That isn’t to say the early Hydra-Matic was flawless. Aside from its cost, it was bulky and quite heavy; we don’t have any precise weight figures for the early units, but they were probably at least 100 lb (45 kg) heavier than a conventional gearbox. Even discounting the inevitable teething problems, Hydra-Matic was also a complicated and fussy device. For it to work as intended, the various brake bands and linkages had to be kept properly adjusted, which required special tools and a certain amount of finesse. Even then, the original Hydra-Matic was never a paragon of smoothness, tending to shift with a distinct thump.

However, the more important thing to most potential buyers was that Hydra-Matic worked, offering the painless two-pedal driving for which so many people had been longing. If the automatic transmission’s commendable efficiency didn’t quite match that of a manual gearbox, that seemed like a small price to pay for the permanent banishment of a generally hated chore. Hydra-Matic also made driving accessible to a whole range of disabled people for whom manual shifting was difficult or impossible. After the war, Oldsmobile would capitalize on that potential by offering “Valiant” models equipped with Hydra-Matic and special driver controls, intended for use by disabled veterans.

Hydra-Matic hood badge on a 1942 Oldsmobile B-44 club coupe © 2009 Aaron Severson
In addition to its obvious convenience, Oldsmobile claimed that Hydra-Matic returned up to 20% better fuel economy than did a standard three-speed transmission. As with the earlier Automatic Safety Transmission, the claimed edge in fuel economy was due less to the transmission and more to the 3.42 or 3.63 axle ratios included with it, which was almost 20% taller (lower numerically) than the 4.10 or 4.30 axles standard with manual shift. Hydra-Matic cars were fairly thirsty in stop-and-go driving, but gentle highway cruising could return up to 19 mpg (12.4 L/100 km), which was excellent for a big American car of this era. (author photo)

The upshot of all this was that Oldsmobile sold about 60,000 Hydra-Matic transmissions for 1940, substantially better than the Automatic Safety Transmission had done in two and a half years. Olds would nearly double those sales for 1941 despite raising the transmission’s price from $57 to a more realistic $100.

Oldsmobile’s exclusivity period ended with the 1940 model year, so for 1941, the automatic transmission became available to other GM divisions. Buick still wanted nothing to do with Hydra-Matic, which Buick chief engineer Charles A. Chayne nicknamed “Hydra-Jerk,” nor did conservative Pontiac chief engineer Benjamin H. Anibal, but Cadillac adopted a new heavier-duty Model 250 Hydra-Matic. The Model 250, most of the particulars of which are described in Kelley’s December 1941 patent disclosure (U.S. Patent No. 2,377,696), functioned much like the Oldsmobile unit, but had greater torque capacity and different first and second gear ratios, courtesy of a new second planetary unit with compounded gears (two distinct but interconnected planetary gear trains) like those of the Automatic Safety Transmission. The second gearset’s rearmost planet carrier handled the job of reintegrating the torque split in third and fourth gears.

Color diagram of the 1941–1945 Model 250 Cadillac Hydra-Matic © 2016 Aaron Severson
Another late addition: a diagram (not to scale) of the 1941–1945 Model 250 Hydra-Matic used in prewar and wartime Cadillacs and a variety of military vehicles. Note that there are two rear sun gears rather than one, both driven at the same speed by the main shaft. The rear gearset also has two ring gears and two sets of planet gears on separate planet carriers; the first carrier also forms the second annulus. Cadillac reverted to a simpler non-compounded rear planetary gearset (like that of the Oldsmobile unit) for 1946. (Author diagram)

About 30% of Cadillac buyers ordered the automatic transmission despite its $125 price tag — almost 10% of the list price of a Series 61 club coupe, the division’s cheapest and most popular 1941 model. The option continued for the 1942 model year, which was cut short by the War Production Board in February 1942 so that manufacturing resources could be redirected to the production of war matériel. By then, the Detroit Transmission Division had delivered almost 215,000 Hydra-Matic transmissions.

1942 Oldsmobile B44 Special Sixty-Six club coupe side © 2009 Aaron Severson
Big chrome badges on either side of the hood of this 1942 Oldsmobile proudly proclaim the presence of Hydra-Matic transmission; by this time, about 45% of new Oldsmobiles were so equipped. Note the dull finish of the grille: Like many 1942 American cars, Oldsmobile’s trim reflected an industry-wide program to reduce the use of aluminum, zinc, chrome, and other strategic materials. (author photo)


Unlike Oldsmobile and Cadillac passenger cars, Hydra-Matic would not cease production during the war. Instead, the automatic transmission would find a whole new application.

When America entered the war in late 1941, the principal U.S. light tank was the M3 Stuart, manufactured by the American Car & Foundry Co. In its initial production form, the M3 weighed 14 tons (12.7 metric tons) and was armed with a 37mm cannon and four 0.30-caliber (7.62mm) machine guns. It was powered by a seven-cylinder Continental W-670 radial engine with 262 gross horsepower (195 kW), providing a reasonably sprightly top speed of 36 mph (58 km/h). Even before the U.S. declaration of war, the M3 was already seeing active duty with the British Army, which nicknamed the tank “Honey” and made extensive use of it in North Africa. By 1942, the M3 would also be in widespread service with the U.S. Army and the United States Marine Corps.

With demand for the Continental radial engine already outpacing supply by mid-1941, some M3s were built with the less-powerful nine-cylinder Guiberson diesel. As an alternative, Cadillac proposed a new M3 variant that would trade the nine-cylinder radial engine for two of the division’s passenger car V-8s. That project, supervised by Cadillac engineer Edward N. Cole (later to become chief engineer of Cadillac and Chevrolet and eventually president of General Motors), used two more-or-less stock 346 cu. in. (5,676 cc) Cadillac L-head V-8 engines, each rated at 148 gross horsepower (110 kW) and each driving one tread via a beefed-up Hydra-Matic transmission.

M5A1 Stuart tank front 3q © 2005 User:Bukvoed (CC BY 2.5 Generic)
An M5A1 Stuart light tank, photographed outside the Yad La’Shiryon Museum in Latrun, Israel. (Photo: “M5A1-Stuart-latrun-2” © 2005 User:Bukvoed; resized and used under a Creative Commons Attribution 2.5 Generic license)

The twin Cadillac engines gave the redesigned tank the same top speed as the M3A1 despite a weight increase of about 5,100 lb (2,313 kg). The Hydra-Matic transmissions not only reduced driver workload (an important consideration in a combat vehicle — particularly tanks, which as a rule are cramped, deafeningly loud, and have dreadful visibility), but also allowed the installation of full dual controls so that the tank could be operated by either the driver or co-driver as needed.

The Army Ordnance Department was duly impressed, so the first production example of the redesigned tank, designated M5 Stuart (Stuart VI in British service), rolled off the Cadillac assembly lines in March 1942. In June, Cadillac created the M8 Howitzer Motor Carriage, which shared the M5’s chassis and powertrain, but had a different turret carrying a 75mm artillery piece. About 1,800 M8s and almost 9,000 M5s and improved M5A1s were built in all, some by Cadillac and some by the tractor manufacturer Massey-Harris.

Although the M5 was fast for a light tank, it soon became painfully apparent that it was too lightly armed for the European theater. Cadillac responded with an enlarged version that retained the dual-engine powertrain, but traded one of the 0.30-caliber (7.62mm) machine guns for a 0.50-caliber (12.7mm) gun and the 37mm cannon for a new 75mm cannon shared with the North American B-25H Mitchell bomber. The new tank, dubbed M24 Chaffee, began entering service in April 1944 and reached frontline units that November. More than 4,300 M24s were built by the end of the war and some remained in service until the late eighties. Late in the war, there were also a number of M24 derivatives sharing its chassis, engines, and automatic transmissions, including the M19 anti-aircraft gun carriage and the M37 and M41 self-propelled howitzers.

M24 Chaffee tank front 3q © 2009 Joe Mabel (CC BY-SA 3.0 Unported)
An M24 Chaffee tank at the Fort Lewis Military Museum. (Photo: “M-24 Chaffee Light Tank 01” © 2009 Joe Mabel; resized and used under a Creative Commons Attribution-ShareAlike 3.0 Unported license)

In 1944, the Cadillac V-8/Hydra-Matic powertrain was adapted for the LVT-3 Bushmaster tracked amphibious landing vehicle, although the LVT-3 was built by Borg-Warner and Graham-Paige rather than any GM division. Hydra-Matic transmissions, though not Cadillac engines, were also used in the Chevrolet T-17E1 and T-17E2 Staghound 4×4 armored cars, the GMC T-18 and T-18E2 Boarhound 8×8 armored cars, and the abortive Chevrolet M38 Wolfhound 6×6 armored car. (None of the armored cars saw any U.S. service except for testing and evaluation.)

In all, around 25,000 wartime Allied military vehicles used the Hydra-Matic transmission, not including passenger cars. Military duty brought only minor design changes to the civilian Hydra-Matic, but the experience pushed the Detroit Transmission Division to resolve most of the transmission’s early issues and demonstrated that Hydra-Matic was fundamentally sound and reasonably reliable despite its complexity. Postwar Cadillac and Oldsmobile advertising would proudly proclaim that Hydra-Matic had been proven in combat.


Hydra-Matic really took off during the postwar boom. Buick and Chevrolet still disdained it, eventually opting to develop their own torque converter automatics based on the latest concepts from the corporate engineers, but Pontiac reluctantly adopted Hydra-Matic in 1948. The transmission was enormously popular despite its high prices, which in 1948 ran to $174.25 on a new Cadillac and $185 on an Oldsmobile or a Pontiac (the latter equivalent to more than $1,600 in 2010 dollars). That year, 73% of Pontiac buyers, 97% of Cadillac buyers, and nearly all Oldsmobile buyers opted for Hydra-Matic.

Hydra-Matic quadrant in a 1949 Pontiac Chieftain © 2010 David Cory (CC BY 2.0 Generic)
Postwar Hydra-Matics now had a N-D-L-R shift pattern and revised gear ratios. Hydra-Matic didn’t have a Park position until the second-generation Controlled Coupling Hydra-Matic arrived in 1956, but with the engine off, the reverse pawl would effectively lock the transmission output shaft, serving the same purpose. (Photo: “Hydramatic Drive” © 2010 David Cory; used under a Creative Commons Attribution 2.0 Generic license)

The Detroit Transmission Division built its 1 millionth Hydra-Matic in January 1949. By then, it was apparent that not only was automatic transmission a major marketing advantage, lacking an automatic was becoming a serious competitive handicap. Despite the scorn of critics like Mechanix Illustrated‘s Tom McCahill, American buyers were more than happy to accept the drawbacks of automatic transmission if it meant not having to shift.

Inevitably, other automakers were soon forced to follow GM’s lead. Ford and Studebaker turned to Borg-Warner to develop three-speed torque converter transmissions while Packard introduced its proprietary Ultramatic in May 1949.

With the ever-growing demand for Hydra-Matic, the Detroit Transmission Division needed more production capacity than the original factory in Detroit could accommodate. Later that year, the division relocated to a new and much bigger plant in Livonia, Michigan, allowing GM to further expand production and offer Hydra-Matic to automakers that couldn’t afford to develop their own automatics.

Remarkably, one of the first non-GM customers was Lincoln-Mercury, which added Hydra-Matic as a Lincoln option in mid-1949; Ford’s new Fordomatic/Merc-O-Matic wasn’t yet ready and in any case didn’t have the torque capacity for the big Lincoln V-8. Lincoln was followed in short order by Nash, which added Hydra-Matic for the 1950 model year, and then Hudson and Kaiser-Frazer, which introduced Hydra-Matic for 1951. Most outside customers didn’t bother to conceal the Hydra-Matic’s GM origins, happy to take advantage of its reputation and name recognition.

Hydra-Matic badge on a 1954 Nash Ambassador Custom sedan © 2009 Aaron Severson
This 1954 Nash Ambassador Custom’s badges proudly announce that it carries the optional Hydra-Matic transmission, which Nash purchased from GM.

Detroit Transmission Division also developed several grades of extra-heavy-duty Hydra-Matic for commercial chassis and heavier vehicles. GMC Truck & Coach introduced Hydra-Matic for some models in 1949 and later extended the option to GMC trucks up to 1½ tons. (Chevrolet made Hydra-Matic optional for some trucks beginning in 1954.) There would also be various other military users, including the M59 armored personnel carrier and, in the early sixties, the M114 tracked command and reconnaissance carrier.

With so many new customers, it took GM less than two years to sell another million Hydra-Matics, making the Hydra-Matic by far the most successful automatic transmission in the world.

Along the way, Hydra-Matic’s internal ratios changed several times, as summarized on the following table. Early Cadillac and military Hydra-Matic transmissions used a compound rear planetary gearset, but to our knowledge, all other iterations used a non-compound rear planetary with a ratio of either 2.53 or 2.63:1.

Hydra-Matic Gear Ratios (Single-Coupling Transmissions)
Gear Early Oldsmobile Early Cadillac/
Postwar Late Postwar
Late Postwar
* Not all civilian users adopted these ratios; those that did (principally Cadillac and Pontiac) adopted them for the 1955 model year.
1st 3.66 3.26 3.82 4.10 3.92
2nd 2.53 2.26 2.63 2.63 2.53
3rd 1.44 1.44 1.45 1.55 1.55
4th 1.00 1.00 1.00 1.00 1.00
Reverse -4.31 -3.77 -4.30 -4.62 -4.54


The Hydra-Matic received a variety of minor changes throughout its life, including revised gearing, new clutches, and several new oil pump designs. For 1951, there was also a new reverse planetary gearset engaged via a hydraulically operated cone clutch. The previous pawl was retained, but now served only as a parking brake.

From an owner standpoint, the most significant revision was the introduction for the 1952 model year of the new Dual-Range Hydra-Matic. The production Dual-Range transmission was designed by corporate transmission engineer Kenneth W. Gage, although the basic idea had been described in Earl Thompson’s patent disclosures as far back as 1934.

As the name implied, the Dual-Range Hydra-Matic now provided two Drive ranges. For the sake of clarity, we’ll describe them as “D4” and “D3,” although depending on the make of the car, “D3” might also be called “DLeft” or “S” (for Super, not Second) while “D4” might be alternatively described as “DRight” or just “D.” In D4, the transmission would shift normally through all four gears. In D3 range, line pressure was applied to hold the 3–4 shift valve closed, just as if the throttle were floored. The other shift valves were unaffected, so the transmission could still shift normally between the three lower gears. However, it wouldn’t shift into fourth until road speed reached the maximum full-throttle upshift point, which, depending on axle ratio and tire size, was typically between 65 and 72 mph (105 and 115 km/h). If you did floor the throttle in D3, the transmission would kick down into second as long as you were below the maximum allowable downshift speed for that gear. (The normal Low range was also retained, but was revised to give second-gear starts at part throttle, a useful feature in slippery conditions where first would cause too much wheelspin.)

Since the maximum speed of the 3–4 upshift speed was the same in both Drive ranges, selecting D3 didn’t make much difference in flat-out acceleration. (You could slightly improve your times by also using Low to delay the two-three upshift.) The additional Drive range was mostly intended to provide greater flexibility in hilly terrain, although being able to force a manual downshift to third was sometimes handy for highway passing.

Dual-Range Hydra-Matic quadrant in a 1952 Pontiac Chieftain Eight station wagon © 2010 Aaron Severson
The shift quadrant of this 1952 Pontiac shows the dual Drive ranges, indicated by the arrows on either side of the “DR” mark. Oldsmobile labeled these positions “DR” and “S” respectively while other manufacturers typically used “D4” and “D3.” Note that there’s still no separate Park position and Reverse is still located below Low. The latter arrangement was deliberately chosen to make it easier to rock a car free of mud or snow, but some experts (including Oscar Banker) argued that the pattern made it too easy to accidentally select Reverse. The new reverse unit Hydra-Matic adopted in 1951 was supposed to prevent that by locking out Reverse except at very low speeds, but in the mid-sixties, GM finally adopted the now-standard PRNDL pattern for all the corporation’s various automatics. (author photo)

In 1952, General Motors signed a licensing agreement with Rolls-Royce that allowed the prestigious British automaker to manufacture its own version of the Dual-Range Hydra-Matic for Bentley and Rolls-Royce passenger cars. GM also continued to supply Hudson, Kaiser, Lincoln, Nash, and (from 1953 to 1955) Willys, bringing annual Hydra-Matic production to more than 700,000 units.

That production was interrupted in August 1953 by a catastrophic fire that completely destroyed the Detroit Transmission plant in Livonia. The fire was one of the worst industrial disasters of its era, causing six deaths and more than $80 million in damage. To replace the Livonia facility, GM arranged to lease Kaiser’s Willow Run factory, a former bomber plant in Ypsilanti that Kaiser-Frazer had acquired in 1945. Kaiser was happy to be rid of the plant; it had been sitting idle since June, when Kaiser transferred production to Toledo following its merger with Willys-Overland. Willow Run was converted to Hydra-Matic production by mid-September and GM bought the plant outright in November for $26 million. While Willow Run was being retooled, Hydra-Matic users briefly had to substitute other transmissions — Dynaflow for Oldsmobile and Cadillac, Powerglide for Pontiac, and Borg-Warner for most non-GM customers. Hydra-Matic deliveries resumed that November.

The original Hydra-Matic was by then nearing the end of its useful life, mostly because customers were becoming less tolerant of its occasionally harsh shifts. Back in 1952, the Transmission Development Group had started work on a thoroughly redesigned second-generation Hydra-Matic, which finally debuted for the 1956 model year. We’ll look at that transmission and its successors in our second installment.

1954 GMC 150 pickup Hydra-Matic badge © 2009 Aaron Severson
The Dual-Range Hydra-Matic became available on GMC light trucks in 1952; GMC’s M135 and M211 series of 6×6 military trucks had adopted it in 1951, mated to a two-speed axle. Hydra-Matic became optional on some Chevrolet trucks in 1954. (author photo)


Earl Thompson left GM in May 1940 and subsequently established his own company, the Earl A. Thompson Manufacturing Company, based in Ferndale, Michigan. In 1963, Thompson received the Elmer A. Sperry Award and the other members of his group (including Beck, Carnegie, Herndon, Kelley, and Rosenberger) received Citations for their work on the original Hydra-Matic. Thompson died in April 1967 at the age of 75.

Three months after Thompson’s resignation, Charles McCuen, who as Oldsmobile general manager had played no small part in bringing the Hydra-Matic to production, was promoted to vice president of engineering, succeeding O.E. Hunt. Seven years later, McCuen became head of GM’s research division, replacing the retiring Charles Kettering. In 1955, he was badly injured in the crash of the turbine-powered Firebird I research vehicle and took early retirement. He died in 1975.

Harold Metzel, who led Oldsmobile’s development work on the production Hydra-Matic, became Oldsmobile chief engineer in January 1951 and the division’s general manager from July 1964 until his retirement in April 1969. During his later years at Oldsmobile, he would oversee the introduction of the Oldsmobile Toronado, GM’s first front-wheel-drive car.

In August 1940, O.K. Kelley became Thompson’s de facto successor as the head of a reorganized corporate transmission development unit. As we’ll see in our next installment, Kelley would play an important role in GM’s subsequent transmission development efforts, including Dynaflow and Powerglide.

Production of the original single-coupling Hydra-Matic eventually reached more than 7 million units, which does not include heavy trucks, military vehicles, or transmissions built under license by Rolls-Royce (for which we don’t have a count). Most non-military users switched to later transmissions by the late fifties, but GMC offered the older Dual-Range Hydra-Matic on some trucks into the 1962 model year. Rolls-Royce continued to use its license-built version of that transmission for the Phantom V and Phantom VI limousines through 1978.

The Hydra-Matic’s impact, however, has extended far beyond even those numbers. As we said at the beginning, the Hydra-Matic was not the first automatic transmission, but it would be hard to deny that it was the first really successful one. The precedents it set shaped powertrain development for the next 60 years.

In our next installment, we’ll take a look at some of GM’s other early automatics along with the second-generation Controlled Coupling Hydra-Matic and the third-generation Roto Hydra-Matic.



Special thanks to Alden Jewell and to Lead Archivist Christo Datini of the GM Media Archive, who contacted us after the original publication of this article and provided a very useful table of original Hydra-Matic production totals.


Our sources on Earl Thompson, Charles McCuen, and the development of Synchro-Mesh and the Automatic Safety Transmission included “Almost Automatic,” Special Interest Autos #20 (January-February 1974), pp. 24-27; John Barach, “Automobile Drivetrain History,” Motor Era, 1999, www.motorera. com/ history/ hist10.htm, accessed 10 May 2010; Buick Motor Division of General Motors Sales Corporation, “Buick 1938,” [brochure, ca. October 1937]; “Deaths,” Astronautics & Aeronautics Vol. 5 (1967), p. 33; George Derby and James Terry White, eds., The National Cyclopædia of American Biography Vol. 53 (New York: J.T. White & Co., 1971), pp. 462–463; Terry B. Dunham and Lawrence R. Gustin, The Buick: A Complete History (An Automobile Quarterly Magnificent Marque Book), Third Edition (Kurtztown, PA: Automobile Quarterly, 1987); Helen Jones Earley and James R. Walkinshaw, Setting the Pace: Oldsmobile’s First 100 Years (Lansing, MI: Oldsmobile Division of General Motors Corporation, 1996); General Motors Corporation, “GM Powertrain: Past, Present, Future,” www.gm. com/experience/ technology/ gmpowertrain/ about/powertrain_history.jsp [now www.gmpowertrain. com], accessed 28 May 2010; “Generations of GM History: McCuen, Charles L.,” Generations of GM History, GM Heritage Center, history.gmheritagecenter. com, accessed 17 May 2010; Philip G. Gott, Changing Gears: The Development of the Automotive Transmission (SAE Historical Series) (Warrendale, PA: Society of American Engineers, 1991); Stan Grayson, “Retrospect: 1938 Oldsmobile LA38,” Motor Trend Vol. 32, No. 11 (November 1980): 114–121; Maurice D. Hendry, Cadillac: Standard of the World: The Complete History (Fourth Edition update by David R. Holls) (Princeton, NJ: Automobile Quarterly, 1990); “Result of Impatience,” The Montreal Gazette 7 February 1931, p. 6; Harvey S. Jacobs, Ferndale, 1918-1943: 25 Years of Progress (Ferndale, MI: Harvey S. Jacobs, 1943); the Old Car Brochures website (oldcarbrochures.org); the Old Car Manual Project (www.oldcarmanualproject. com); Scott Oldham and Michael Lamm, “Happy 100th!” Popular Mechanics Vol. 173, No. 5 (May 1996), pp. 47–59; Oldsmobile Division of General Motors Sales Corporation, “Oldsmobile Six and Eight” [brochure], ca. October 1937, and “Oldsmobile Series 60 – Series 70 – Series 80” [brochure], ca. October 1938; Alfred P. Sloan with John McDonald, My Years with General Motors (Garden City, NY: Doubleday, 1964); Earl A. Thompson, “Automatic Gear-Shifting Mechanism for Sliding Gear Transmission,” U.S. Patent No. 1,435,430, filed 9 March 1918, issued 14 November 1922, reissued 25 September 1928; assignor to General Motors Corporation, “Automatic Gear Shifting Mechanism,” U.S. Patent No. 2,101,825, filed 9 October 1923, issued 7 December 1937; “Transmission,” U.S. Patent No. 1,827,960, filed 15 April 1925, issued 20 October 1931; assignor to General Motors Corporation, “Power Transmission Mechanism,” U.S. Patent No. 1,876,098, filed 15 April 1925, issued 6 September 1932; assignor to General Motors Corporation, “Transmission,” U.S. Patent No. 1,888,640, filed 15 April 1925, issued 22 November 1932; assignor to General Motors Corporation, “Clutch Mechanism for Transmissions,” U.S. Patent No. 1,854,281, filed 2 August 1926, issued 19 April 1932; assignor to General Motors Corporation, “Change Speed Gearing with Automatic Overdrive,” U.S. Patent No. 2,285,760, filed 6 March 1933, issued 9 June 1942; assignor to General Motors Corporation, “Change-Speed Transmission and Control,” U.S. Patent No. 2,195,605, filed 8 October 1934, issued 2 April 1940; assignor to General Motors Corporation, “Change-Speed Mechanism and Control,” U.S. Patent No. 2,193,304, filed 16 October 1935, issued 12 March 1940; assignor to General Motors Corporation, “Clutch and Gearing Control,” U.S. Patent No. 2,362,418, filed 15 March 1937, divided 14 February 1940, issued 7 November 1944; and assignor to Earl A Thompson Mfg. Company, “Vehicle Visor,” U.S. Patent No. 2,492,074, filed 16 July 1946, issued 20 December 1949; and Angelo Van Bogart, Cadillac: 100 Years of Innovation (Iola, WI: Krause Publications, 2003).

Additional information on the Hydra-Matic came from Thomas Bonsall, “The Great Hydra-Matic Fire,” February 1999, www.autotran. us/ TheGreatHydraMaticFire.html, accessed 10 June 2010; Ray T. Bohacz, “Mechanical Marvels: Shiftless Driving: Oldsmobile introduces the fully automatic transmission,” Special Interest Autos #180 (November-December 2000), pp. 54-56, and “Mechanical Marvels: Shiftless Pleasure: The 1956 General Motors Hydra-Matic Transmission,” Hemmings Classic Car #35 (August 2007), pp. 68–71; Griff Borgeson, “Cadillac Motor Trial,” Motor Trend Vol. 3, No. 11 (November 1951), reprinted in Cadillac Automobiles 1949-1959, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1990), pp. 14–16; Arch Brown, “1949 Pontiac Eight: ‘The Most Beautiful Thing on Wheels,'” Special Interest Autos #111 (May-June 1989), reprinted in The Hemmings Motor News Book of Pontiacs: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001), pp. 18–26; “SIA comparisonReport: 1949 Cadillac vs. 1949 Oldsmobile 98: Similar But Different,” Special Interest Autos #149 (September-October 1995), reprinted in The Hemmings Book of Oldsmobiles: driveReports from Hemmings Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001), pp. 34–41; and “1948 Cadillac 61: The First Shall Be Last,” Special Interest Autos #171 (May-June 1999), reprinted in The Hemmings Motor News Book of Cadillacs: driveReports from Hemmings Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2000), pp. 52-59; Arch Brown, Richard M. Langworth, and the Auto Editors of Consumer Guide, Great Cars of the 20th Century (Lincolnwood, IL: Publications International, Ltd., 1998); Martin Bunn, “Gus and Joe Go to the Show,” Popular Science Vol. 137, No. 5 (November 1940), pp. 136–140, 241–242; the Cadillac & LaSalle Club Modified Chapter website (www.modifiedcadillac.org); Cadillac Motor Car Division of General Motors Corporation, Service Department, “Cadillac…From Peace to War” [brochure], 1943; Cadillac Shop Manual for 1941 (Detroit, MI: General Motors Division, 1940); Cadillac Shop Manual Supplement for 1946 (Detroit, MI: General Motors Division, 1945); Cadillac Shop Manual Supplement for 1951 (Detroit, MI: General Motors Corporation, 1951); Cadillac Shop Manual Supplement for 1955 (Detroit, MI: Cadillac Motor Car Division, 1955); Cadillac Hydra-Matic Drive Shop Manual: Shop Manual Covering Construction, Operation, Adjustment and Repair of the Hydra-Matic Drive as Used on the 1941 Cadillac Cars (Detroit, MI: General Motors Division, 1941); “Cadillac: Standard of the World 1946” [brochure], December 1945; and “Data Book” [salesman’s guide], 13 September 1940; William L. Carnegie, assignor to General Motors Corporation, “Variable Speed Control,” U.S. Patent No. 2,221,393, filed 1 July 1938, issued 12 November 1940, and “Compound Planetary Gear Train,” U.S. Patent No. 2,606,459, filed 6 December 1947, issued 12 August 1952; Alan Chanter, “Landing Vehicle Tracked,” World War II Database, ca. 2012, ww2db. com/ vehicle_spec.php?q=301, accessed 20 September 2015; Chevrolet Motor Division of General Motors Corporation, “1954 Chevrolet Advance-Design Trucks: For Loads of Value: [brochure 1,000 M], October 1953; “Continental Road Test No. 2C/50 — The Cadillac V.8 S.62,” The Motor 22 March 1950, reprinted in reprinted in Cadillac Automobiles 1949-1959, pp. 10-12; John Day, The Bosch Book of the Motor Car: Its evolution and engineering development (New York: St. Martin’s Press, 1976); Detroit Transmission Division of General Motors Corporation, “Detroit Transmission Division Welcomes You to the Home of Hydra-Matic” [flyer, ca. 1958], via GM Heritage Center; “Detroit Transmission Division, General Motors Factory,” Emporis, n.d., www.emporis. com/ buildings/ 334613/ detroit-transmission-division- general-motors-factory-detroit-mi-usa, accessed 19 September 2015; David Edwards, Antique Automatic Transmission Parts, 21 July 2002, www.autotran.us, accessed 15 May 2010; J. Kelly Flory, Jr., American Cars, 1946–1959: Every Model, Year By Year (Jefferson, NC: McFarland & Company, Inc., Publishers, 2008); GMC Truck & Coach Division, General Motors Corporation, “GMC Pickups” [brochure Adv. 430 6-61, June 1961]; the GM Heritage Archive (gmheritagecenter. com/ gm-heritage-archive/); T. Grace, Automatic Transmission Service Guide (Union, NJ: Lincoln Technical Institute, September 1966); Ken Gross, “1942 Oldsmobile B-44,” Special Interest Autos #40 (May-July 1977), reprinted in The Hemmings Book of Oldsmobiles, pp. 20–24; John Gunnell, ed., Standard Catalog of Pontiac 1926–2002 (Iola, WI: Krause Publications, 2002); Roger Huntington, “The Great Transmission Controversy: Coupling vs. Converter,” Car Life Vol. 10, No. 2 (March 1963), pp. 18-21; “History of the Hydra-Matic transmission, 1932–1967,” www.autotran. us/ hydramatic_history.html, accessed 10 October 2015; David D. Jackson, “The U.S. / American Automobile Industry in World War II / WW II: Detroit Transmission Division of General Motors Corporation in World War Two / WWII,” 19 February 2015, usautoindustryworldwartwo. com/ General%20Motors/ detroit-transmission.htm, accessed 20 September 2015; Robert Johnson, “Cadillac 62 Sedan,” Motor Life August 1954, reprinted in Cadillac Automobiles 1949-1959, pp. 39–40; Robert C. Juvinall and Kurt M. Marshek, Machine Component Interrelationships, Fifth ed. (Hoboken, NJ: John Wiley and Sons, Ltd., 2011); Oliver K. Kelley, assignor to General Motors Corporation, “Combination Fluid Turbo Clutch and Variable Speed Gearing,” U.S. Patent No. 2,176,138, applied 5 February 1937, issued 17 October 1939; “Fluid Flywheel Gearing Arrangement,” U.S. Patent No. 2,211,233, applied 10 April 1939, issued 13 August 1940; and “Transmission Drive,” U.S. Patent No. 2,377,696, filed 15 December 1941, issued 5 June 1945; Al Kidd, “’55 Olds Super 88,” Motor Trend Vol. 7, No. 5 (May 1955), reprinted in Oldsmobile Automobiles 1955-1963, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1989), pp. 5–6, 55; Ted Koopman, “Speed Age Tests the 1952 Pontiac,” Speed Age March 1952, reprinted in Pontiac Limited Edition Extra 1949-1960, ed. R.M. Clarke (Cobham, England: Brooklands Books Ltd., ca. 1999), pp. 14–17; Julian P. Leggett, “Testing the Cadillac Series 62,” Science and Mechanics October 1952, reprinted in Cadillac Automobiles 1949-1959, pp. 24–26; Jim Lodge, “Pontiac Star Chief,” Motor Trend Vol. 6, No. 5 (May 1954), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 32–34; Terry McGean, “GM Hydra-Matic,” Hemmings Motor News September 2006; Pete Molson and Walt Woron, “Pontiac Eight,” Motor Trend Vol. 5, No. 5 (May 1953), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 23–26; Mike Mueller, Pickup Trucks (Osceola, WI: Motorbooks, 2003); L.H. Nagler, “How Your Car Shifts for Itself,” Popular Mechanics Vol. 89, No. 5 (May 1948): 102–106, 264, 268, 272; ‘Nailhed,’ “The Night Shift,” Nailhed.com, June 2014, www.nailhed. com/ 2014/06/ the-night-shift.html, accessed 25 June 2014; Eric Nielssen, “Six Luxury Cars: a view from the Automotive Engineering Side,” Car and Driver Vol. 11, No. 1 (July 1965), 26–31, 62–65, 75; Jan P. Norbye and Jim Dunne, Pontiac 1946-1978: The Classic Postwar Years (Osceola, WI: Motorbooks International, 1979); “Oldsmobile,” Automotive Industries Vol. 83 (1 October 1940), p. 302; Oldsmobile Motor Division of General Motors Corporation, “For 1952 Oldsmobile Rockets to New Highs” [1952 model year brochure], 1952; “Hydra-Matic Drive: Designed, Developed and Introduced by Oldsmobile” [brochure, ca. Oct. 1940], “Oldsmobile Hydra-Matic Drive” [brochure, ca. 1946], and “Oldsmobile’s Exclusive Hydra-Matic Drive” [brochure, ca. October 1939], via GM Heritage Center; and “Driving Sensation of the Year! Whirlaway with Hydra-Matic Drive” [brochure], 1948; 1940 Oldsmobile Shop Manual (Lansing, MI: Oldsmobile Division of General Motors Sales Corporation, 1939); “Oldsmobile Six and Eight” [1938 model year brochure], 1937; “Oldsmobile: Styled to Lead, Built to Last” [1941 model year brochure], 1940; “New Oldsmobile” [1946 model year brochure], 1945]; “The One BIG New Engineering Feature in the 1940 Cars!” [advertisement], Popular Science Vol. 72, No. 6 (December 1939), p. 5; Terry Shea, “A Valiant Effort! 1947 Oldsmobile 66,” Hemmings Classic Car #118 (July 2014); Bill Siuru, “Hydra-Matic celebrates its 60th birthday,” Old Cars Weekly 1 November 1999; “Six Luxury Cars: A subjective, seat-of-the-pants evaluation by the editors,” Car and Driver Vol. 11, No. 1 (July 1965): 23–25, 62–65; “Sperry Award Recipients,” The Elmer A. Sperry Award website, sperryaward. org/recipients.htm, accessed 24 September 2015; “Streamliners and Chieftains by Pontiac,” The Motor 16 March 1949, reprinted in Pontiac Limited Edition Extra 1949-1960, p. 5; “Testing the New Dual-Range H-M Pontiac,” Motor Trend Vol. 4, No. 4 (April 1952), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 19–21; “The Autocar Road Tests No. 1447: Cadillac Series 62 Saloon,” The Autocar 9 November 1951, reprinted in Cadillac Automobiles 1949-1959, pp. 17–19; “The High-Compression Cadillacs,” The Motor 2 March 1949, reprinted in Cadillac Automobiles 1949-1959, pp. 5–7; Earl A. Thompson, assignor to General Motors Corporation, “Change Speed Gearing and Control,” U.S. Patent No. 2,204,872, filed 1 April 1938, issued 18 July 1940; “Fluid Coupling Rotor,” U.S. Patent No. 2,357,295, filed 5 February 1940, issued 5 September 1944; and “Rotary Hydraulic Coupling of the Turbine Type,” U.S. Patent No. 2,430,258, filed 17 February 1941, issued 4 November 1947; William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975), pp. 558-574; Robert Temple, “Transmissions and Drive Lines (Know Your Car Part Two),” Motor Trend Vol. 15, No. 1 (January 1963), pp. 54-59; United Motors Service Division, The Hydra-Matic Transmission 1946-1955: On-the-Car Adjustment Service Manual (Detroit, MI: United Motors Service Division, 1956); Josiah Work, “This Is Not Your Grandpa’s Oldsmobile: 1949 Rocket 88,” Special Interest Autos #139 (January-February 1994), reprinted in The Hemmings Book of Oldsmobiles, pp. 26–33; Walt Woron, “Pontiac Motor Trial: 3500-Mile Motor Trial Proves Fine Family Car Tops in Town and on the Open Road,” Motor Trend Vol. 3, No. 9 (September 1951), reprinted in Pontiac Limited Edition Extra 1949-1960, pp. 10–13, and “Road Testing the 50th Anniversary Cadillac,” Motor Trend Vol. 4, No. 9 (September 1952), reprinted in Cadillac Automobiles 1949-1959, pp. 20–23, 33; and Walt Woron and Pete Molson, “Cadillac: America’s Favorite Luxury Car,” Motor Trend Vol. 5, No. 5 (May 1953), reprinted in Cadillac Automobiles 1949-1959, pp. 34–37; and emails to the author from Christine Crawford, 23–25 June 2014. Production figures for passenger car Hydra-Matics came from a table provided by Christo Datini, General Motors Media Archive, General Motors Heritage Center (email to author, 1 June 2010). Prof. John D. Kelly later helped us to sort out some technical points in emails to the author, 7 to 8 March 2017.

Additional information on Reo’s Self-Shifter came from David Traver Adolphus, “Almost Classic: 1935 Reo Royale 75,” Hemmings Classic Car #36 (September 2007), pp. 50–55; the Auto Editors of Consumer Guide, “How Reo Cars Work,” HowStuffWorks.com, 15 June 2007, auto.howstuffworks. com/ reo-cars.htm, accessed 15 May 2010; Horace T. Thomas, John Bethune, and Albert B. Hays, assignors to Reo Motor Car Co., “Transmission Mechanism,” U.S. Patent No. 1,885,156, filed 3 April 1931, issued 1 November 1932; Horace T. Thomas and John Bethune, assignors to Reo Motor Car Co., U.S. Patent No. 1,988,636, “Transmission Mechanism,” filed 12 February 1932, granted 22 January 1935; Horace T. Thomas and Albert B. Hays, assignors to Reo Motor Car Co., U.S. Patent No. 1,988,466, “Transmission Mechanism,” filed 17 October 1932, granted 22 January 1935; Horace T. Thomas, assignor to Reo Motor Car Co., U.S. Patent No. 1,950,580, “Automatic Variable Speed Transmission,” filed 16 January 1933, granted 13 March 1934, and U.S. Patent No. 1,950,581, “Variable Speed Transmission Mechanism,” filed 30 January 1933, granted 13 March 1934; Horace T. Thomas and Albert B. Hays, assignors to Reo Motor Car Co., U.S. Patent No. 2,016,350, “Transmission Mechanism,” filed 18 September 1933, granted 8 October 1935, and U.S. Patent No. 2,038,812, “Transmission Mechanism,” filed 8 June 1934, granted 28 April 1936; Reo Motor Car Co., “Gearshifting ABOLISHED by Reo–Not Merely Made Easier” [advertisement], Popular Science Vol. 125, No. 1 (July 1934), p. 11, and “This Amazing Invention Abolishes Gear-Shifting by Hand in the New 1934 REO” [advertisement], Popular Science Vol. 124, No. 4 (May 1934), p. 8.

Additional information on the Daimler Fluid Flywheel came from Kevin Bennett, “How to drive a Wilson Pre-selector,” My Daimlers and Lanchesters, n.d., daimlerandlanchester. com/ how-to-drive-a-pre-selector/, accessed 12 September 2015; Tony Cooper, “Daimler and Lanchester History and Models, 1925-1938,” n.d., www.daimler. co.uk, accessed 30 August 2015; “Daimler History,” Unique Cars and Parts, n.d., www.uniquecarsandparts.com. au, accessed 12 September 2015; Laurence H. Pomeroy, “Power Transmission Mechanism,” U.S. Patent No. 1,975,700, filed 32 May 1932, issued 2 October 1934; Laurence H. Pomeroy and Alfred Blundell, assignors to the Daimler Company Ltd., “Power Transmission Mechanism,” U.S. Patent No. 1,914,289, filed 21 April 1932, issued 13 June 1933; Harold Sinclair, “Power Transmission Mechanism and Clutch,” U.S. Patent No. 1,831,770, filed 16 March 1929, issued 10 November 1931; “Hydraulic Transmission Gear and Brake,” U.S. Patent No. 1,859,607, filed 17 June 1929, issued 24 May 1932; “Hydraulic Coupling,” U.S. Patent No. 1,937,364, filed 8 January 1931, issued 28 November 1933; “Hydraulic Coupling,” U.S. Patent No. 1,963,720, filed 8 January 1931, issued 19 June 1934; “Power Transmission Mechanism,” U.S. Patent No. 1,978,172, filed 19 September 1931, issued 23 October 1934; and “Power Transmission Mechanism,” U.S. Patent No. 2,102,755, filed 19 September 1931, issued 21 December 1937; and “Vulcan-Sinclair Fluidrive Makes headway,” The Commercial Motor 16 August 1946, p. 43, archive.commercialmotor. com, accessed 15 September 2015.

Additional information on the M-3, M5, and M24 tanks and other Hydra-Matic-equipped military vehicles came from “AUTOS: The New Generation,” TIME 5 October 1959, www.time. com, accessed 14 July 2010; Joe Baugher, “North American B-25G Mitchell,” 10 March 2000, www.joebaugher. com/ usaf_bombers/ b25_13.html, accessed 17 May 2010; “North American B-25H Mitchell,” 11 March 2000, www.joebaugher. com/ usaf_bombers/ b25_15.html, accessed 17 May 2010; Alan Chanter, “Landing Vehicle Tracked,” World War II Database, ca. 2012, ww2db. com/ vehicle_spec.php?q=301, accessed 20 September 2015; “Chevrolet Staghound T-17E2 (AA),” n.d., ramrao2.tripod. com/ staghound/, accessed 20 September 2015; “Cole, Edward N.,” Generations of GM, GM Heritage Center, n.d., history.gmheritagecenter. com/wiki/ index.php/ Cole,_Edward_N., accessed 14 July 2010; Chris Conners, The AFV Database, “Armored Car T17E1 Staghound,” 1 May 2012, afvdb.50megs. com/ usa/ t17e1.html, accessed 21 September 2015; “Armored Command and Reconnaissance Carrier M114,” afvdb.50megs. com/usa/ m114.html, accessed 21 September 2015; “Armored Personnel Carrier M59,” 3 December 2002, afvdb.50megs. com/usa/ apcm59.html, accessed 21 September 2015; “Light Armored Car M38 Wolfhound,” 2 April 2014, afvdb.50megs. com/ usa/ m38wolfhound.html, accessed 21 September 2015; “Light Tank M3 Stuart,” 22 July 2007, afvdb.50megs. com/ usa/ m3stuart.html, accessed 17 May 2010; “Light Tank M5 Stuart,” 1 June 2009, afvdb.50megs. com/ usa/ m5stuart.html, accessed 21 September 2015; “Light Tank M24 Chaffee”(29 May 2007, afvdb.50megs. com/ usa/ m24chaffee.html, accessed 21 September 2015; “105mm Howitzer Motor Carriage M37,” 8 March 2008, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; “155mm Howitzer Carriage M41,” 15 October 2014, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; and “Twin 40mm Gun Motor Carriage M19,” 24 April 2014, afvdb.50megs. com/usa/ 40mmgmcm19.html, accessed 22 September 2015; the Editors of Publications International, Ltd., “M-3 Stuart (Honey)/M-5 Light Tank,” HowStuffWorks.com, 17 November 2007, science.howstuffworks. com/ m-3-stuart-honey-m-5-light-tank.htm, accessed 17 May 2010, and “M-24 Chaffee Light Tank,” HowStuffWorks.com, 14 November 2007, science.howstuffworks. com/ m-24-chaffee-light-tank.htm, accessed 17 May 2010; Shawn A. Fisher, GURPS WWII: Dogfaces (Austin, TX: Steve Jackson Games, 2003), p. 78; Martin Foray, “M114 CRV Command and Reconnaissance Vehicle,” Military Factory, 3 August 2015, www.militaryfactory. com/ armor/ detail.asp?armor_id=108, accessed 21 September 2015; and “M-59 Armored Personnel Carrier,” and “M-135 Series 2 1/2-ton, 6×6 Trucks,” Olive-Drab.com, n.d., olive-drab.com, accessed 22 September 2015; the Historical Forces Association 9th Division Infantry website (www.easy39th.com); U.S. Department of the Army, Light Tank M24 (Department of the Army Technical Manual TM 9-729) (Washington, DC: U.S. Government Printing Office, May 1951); and Ordnance Field and Depot Maintenance: 301MG and 303M Hydra-Matic Transmissions (Department of the Army Technical Manual TM 9-8025-2) (Washington, D.C.: U.S. Government Printing Office, 18 June 1957); and U.S. War Department, Ordnance Maintenance: Hydra-Matic Transmission and Propeller Shafts for Light Tanks M5, M5A1, and 75-MM Howitzer Carriage (War Department Technical Manual TM 9-1727C (Washington, DC: U.S. Government Printing Office, 5 February 1943).

Other background information came from David Traver Adolphus, “Cast-Iron Wonder – 1931 AE Sport Coupe,” Hemmings Classic Car #21 (June 2006), pp. 28–35; John O. Almen, assignor to General Motors Corporation, “Control for Toric Friction Transmission,” U.S. Patent No. 2,045,558, filed 20 December 1934, issued 23 June 1936; John O. Almen and John Dolza, assignors to General Motors Corporation, “Hoop Governor with Double Acting Weight,” U.S. Patent No. 1,984,006, filed 11 July 1932, issued 11 December 1934; John O. Almen and Winfield D. Gove, assignors to General Motors Corporation, “Friction Transmission Control Mechanism,” U.S. Patent No. 2,073,134, filed 7 May 1936, issued 9 March 1937; John O. Almen and Harry Hawkins, assignors to General Motors Corporation, “Control for Variable Speed Power Transmissions,” U.S. Patent No. 2,131,157, filed 9 March 1931, issued 27 September 1938; William C. Anderson, “Charles A. 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Banker, assignor to New Products Corporation, “Variable Speed Transmission,” U.S. Patent No. 1,937,503, filed 3 September 1931, issued 5 December 1933; “Clutch Mechanism,” U.S. Patent No. 2,042,454, filed 19 March 1932, issued 2 June 1936; “Automatic Change Speed Transmission,” U.S. Patent No. 1,996,790, filed 3 November 1932, issued 9 April 1935; “Change Speed Transmission,” U.S. Patent No. 1,985,884, filed 14 December 1932, issued 1 January 1935; “Planetary Transmission Mechanism,” U.S. Patent No. 2,005,726, filed 29 June 1933, issued 25 June 1935; “Change Speed Transmission,” U.S. Patent No. 2,077,387, filed 16 July 1934, issued 20 April 1937; “Clutch Mechanism,” U.S. Patent No. 2,104,014, filed 16 July 1934, issued 4 January 1938; “Automatic Transmission,” U.S. Patent No. 2,199,095, filed 13 October 1934, issued 30 April 1940; “Change Speed Transmission,” U.S. Patent No. 2,140,502, filed 30 November 1934, issued 20 December 1938; “Automatic Transmission,” U.S. Patent No. 2,171,534, filed 29 May 1935, issued 5 September 1939; “Automatic Transmission,” U.S. Patent No. 2,262,747, filed 18 September 1936, issued 18 November 1941, reissued 18 May 1943; and “Automatic Transmission,” U.S. Patent No. 2,237,297, filed 15 September 1937, issued 8 April 1941; Oscar H. 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Kettering, “Business Needs a New Broom,” The Rotarian May 1931, pp. 6–8, 51–54, and “Hurdles to Jump for Inventors,” Popular Mechanics Vol. 52, No. 6 (December 1929), pp. 954–959; Timothy L Krantz, “Dynamics of a Split Torque Helicopter Transmission,” U.S. Army Research Laboratory, NASA Technical Memorandum 106410/Army Research Laboratory Memorandum–ARL–TRL–291, June 1994, ntrs.nasa. gov/ archive/ nasa/casi.ntrs.nasa.gov/ 19940032949.pdf, accessed 16 September 2015; Michael Lamm, “Model A: The Birth of Ford’s Interim Car,” Special Interest Autos #18 (August-October 1973), reprinted in The Hemmings Book of Prewar Fords: driveReports from Special Interest Autos Magazine, eds. Terry Ehrlich and Richard Lentinello (Bennington, VT: Hemmings Motor News, 2001), pp. 12–21; Richard M. Langworth, Kaiser-Frazer, the Last Onslaught on Detroit: An Intimate Behind the Scenes Study of the Postwar American Car Industry (Automobile Quarterly Library Series) (Boston, MA: E.P. 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Pomeroy and Alfred Blundell, assignors to the Daimler Company Ltd., “Power Transmission Mechanism,” U.S. Patent No. 1,914,289, filed 21 April 1932, issued 13 June 1933; Arthur Pound, The Turning Wheel: The Story of General Motors Through Twenty-Five Years 1908–1933 (Garden City, NY: Doubleday, Doran & Co., Inc., 1934); George Schmidt, “Which Hand?” White Triangle News May-June 1979, pp. 54-55; Michael Sedgwick, Classic Cars of the 1930’s and 1940’s, 2nd ed. (Twickenham, England: Tiger Books International PLC, 1997), and Classic Cars of the 1950’s and 1960’s, 2nd ed. (Twickenham, England: Tiger Books International PLC, 1997); Edwin Storm’s Free Car Brochures website at the Old Car Manual Project (storm.oldcarmanualproject. com); Thomas L. and Thomas J. Sturtevant, “Clutch Device Power Transmitting Mechanism,” U.S. Patent No. 766,551, filed 5 January 1904, issued 2 August 1904, and “Clutch Device,” U.S. Patent No. 766,552, filed 16 February 1904, issued 2 August 1904; Thomas L. and Thomas J. Sturtevant, assignors to Sturtevant Mill Company, “Power-Transmitting Mechanism,” U.S. Patent No. 784,285, filed 10 December 1904, issued 7 March 1905, and “Power-Transmitting Mechanism,” U.S. Patent No. 797,383, filed 10 December 1904, issued 15 August 1905; Rich Taylor, “Boss Kett’s Dog: 1923 Chevrolet Copper-Cooled,” Special Interest Autos #30 (September-October 1975), pp. 44–51; Pat Tobin, “Half-Hour History of MoPar’s Fluid Drive,” Special Interest Autos #116 (April 1990), pp. 40-45; Vincent Tocco, Jr., “Fluid Drive History,” American Blower, n.d., americanblowercorp. com, accessed 15 May 2010; Hans Tore Tangerud’s Autoblog website (www.lov2xlr8.no); and Vulcan Werke Hamburg, Stettiner Maschinenbau Actiengesellschaft, “Flüssigkeitsgetriebe zur Arbeitsübertragung zwischen benachbarten Wellen mittels treibender und getriebener Turbinenräder,” DRP Nr. 238804, filed 24 June 1905, issued 30 September 1911.

The typeface used in this article’s author-created tables, diagrams, and other graphics is Liberation Sans, one of the Liberation Fonts (version 2.00.1 or later), which are copyright © 2012 Red Hat, Inc., used under the SIL Open Font License, Version 1.1. Liberation is a trademark of Red Hat, Inc. registered in U.S. Patent and Trademark Office and certain other jurisdictions. Red Hat is a trademark of Red Hat, Inc., registered in the United States and other countries.

Our inflation estimates were based on the United States Bureau of Labor Statistics Inflation Calculator, data.bls. gov/ cgi-bin/cpicalc.pl. Please note that the inflation figures cited in the text are approximate and are provided solely for general reference — this is an automotive history, not a treatise on the historical value of money, and nothing in this article should be taken as financial advice of any kind!



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  1. I very much enjoyed the first installment of the Hydramatic story. However, I think Henry Ford deserves a little more credit for the basic blue print of a mass produced planetary gearbox. The Model T which debuted in 1908 had the bands, clutch pack and planetary gear sets which are easily recognized in modern automatics.

    1. Henry Ford didn’t invent the planetary gearset — epicyclic gear trains were used for a whole variety of industrial purposes in the 19th century — but he was definitely their most enthusiastic proponent for automotive use (and he also did an enormous amount of work with planetary gears for tractors). I don’t know how much of a direct link there were between the Model T and later planetary gearsets (including the various preselectors, most of which were invented in the 1918-1920 period), but certainly any automotive engineer would have been aware of its design.

      Ironically, Ford’s own efforts to develop an automatic transmission pretty much came to naught; he was keen to include automatic with his "X car" (which would also have had an X-8 engine), but he couldn’t make it work. Ford Motor Company ended up turning to Borg-Warner to develop Ford-O-Matic. Even more ironically, Howard Simpson, who worked very closely with Ford at Fordson (the tractor division) and at Ford on planetary gears, subsequently designed the Simpson gearset used variously by Cruise-O-Matic, TorqueFlite, and Turbo Hydramatic.

    2. I always wonder why Ford didn’t develop an early automatic. As you stated, they already had the transmission design, all that was needed was to hire a good fluid power engineer or two to work out the automation.

      1. Well, Ford did do the Liquamatic, optional on 1942 Lincolns and Mercurys, although by all accounts it didn’t work out well. Interestingly, the car Henry Ford wanted to succeed the Model T was supposed to have automatic transmission as well as the peculiar X-8 engine, but neither worked out. I have no details about the transmission design, but I have to wonder if it was another case of Henry Ford I having a strong fixed idea that he refused to relinquish even when it proved to be impractical. (For Ford prior to WW2, a crucial component of any engineering change was getting Henry Ford to agree to not only the idea, but also the method of achieving it. Ford had a variety of talented engineers whose opinions and suggestions he was not at all interested in hearing.)

        I’m starting to feel that I’ve done everyone a disservice in glossing over the mechanics of the hydraulic control systems for the Automatic Safety Transmission and the early Hydra-Matic, giving the mistaken impression that the controls were a trivial part of the system. That’s really not so — as the text says, the hydraulic controls even of the earliest Hydra-Matic formed a fairly complex analog computer, combining inputs from several different sources to make each shifting ‘decision.’ Developing a system that would perform all the necessary actions at the appropriate times was not a simple or easy task and at least at GM happened in stages over a six- or seven-year period.

        Today, yes, a skilled engineer could probably sit down and design a basic analog hydraulic control system on a home computer, but that’s with the benefit of 80 years of design precedent and a mountain of readily accessible technical literature on the subject. In the 1930s, this was new territory and the designers and inventors were to a large extent making it up as they went along. At the time, even hydraulic brakes were a relatively recent innovation and a hydraulic brake system only has to transmit motion; there’s no logic or computation involved. (And before anyone says it, yes, as Oscar Banker and others strove to demonstrate, it was also possible to design an automatic transmission that would not use hydraulics at all, but a purely mechanical analog computer is not less complex than a hydraulic one — potentially quite the opposite.)

        So, to be very clear: The planetary transmission in a Model T Ford was NOT eight-tenths of an automatic transmission, even a fairly simple two-speed automatic like the 1953-vintage Powerglide, any more than a hang glider is almost a B-17 because they both have wings.

    3. Yet zero automatic controls or shift schedule. Yes, the T was revolutionary in 1909. Not so in 1927.

    4. The planetary system is not what actually seperates an automatic transmission from a gearbox. The implication of gear train components and there progression in automotive history is an entirely different subject. The inventor of the synchro mesh is allways noted on the conception of the synchro, but it’s just a side note to his connection to the development of the hydro matic inside the walls of general motors. As a side note to that side note the article should reference it’s battle testing inside tanks and trucks during the war effort before it’s return to being developed for the automotive industry

      1. Er, all of these things are mentioned in the article at some length.

  2. The Continental R-670 is not a rotary (Wankel) engine, but a radial engine.

    1. Whoops — a slip of the keyboard. Thanks!

    2. Sorry, I didn’t mean to click on the thumbs down icon, happened by accident. Just wanted to clarify.

    3. The original rotary engine was actually a radial engine in which the crankshaft was stationary and the cylinders rotated (Gnome was a big name in rotary engines at the beginning of aviation).

  3. Great article guys!! Keep it coming.

  4. I had vaguely heard of the fire that destroyed the Hydramatic plant in 1953, but this excellent article got me wondering about it. After a bit of searching, I found an interesting photo spread in the November 2, 1953 issue of Life magazine on page 102. It brings home the enormous industrial enterprise and effort that went into the Hydramatic.

    I’m already anticipating next week’s installment…

    1. The Willow Run plant that GM bought to replace the Livonia factory was truly vast. It was built during the war to build heavy bombers. At its peak in 1944, it was churning out 25 B-24 Liberators — four-engine heavies much more complicated than any contemporary car — every day. Kaiser-Frazer leased it after the war, although it had far more capacity than they needed.

      1. It’s still there, and still vast. Note the scale; Willow Run is roughly 4 sq. mi., including runways.

        Willow Run is both a busy freight airport and a GM transmission plant (or [i]was[/i], I’m not sure of Willow Run’s role in New G.M.).

        On a semi-related note, Connie Kalitta’s successful air freight operation is based at Willow Run. Many will recognize his name from drag racing circles.

        In addition to transmissions, Willow Run was the final assembly point for various GM vehicles. For anyone in the Ypsi area, I highly recommend a visit—the grounds are pretty well wide open, since it’s a freight airport. Kind of surreal.

  5. I recall from reading an autobiography by W.O. Bentley that he prided himself on his engines having great “flexibility,” the ability of the motorcar from a standing stop to start off and accelerate smoothly to top speed in high gear.

    In this series, will you cover the Powerglide as well? My grandparent’s ’66 Tempest had one of these (along with the OHC 6 you recently covered), and I still remember the disconnected feeling that came with pressing the accelerator and waiting for things to start happening.

    1. It’s easier to get away without shifting with very large engines — lots of low-end torque. (Given Bentley’s penchant for 4.5-liter fours, he clearly understood that!)

      I’ll touch on Powerglide a bit next week. The A-body Tempest did not use Powerglide, though. The Tempest, Special, and F-85/Cutlass used a different two-speed automatic, which Buick called Super Turbine 300. It was developed by the corporate Engineering Staff to replace the different automatics used by the Y-bodies. It worked a lot like Powerglide, but it wasn’t the same transmission.

      1. Powerglide trans was designed by Bill Wallace head of hydromatic Toledo Ohio with a a converter with variable stator vains in the converter where my dad worked with Bill in the 50 to late 80s

        1. Powerglide went through several distinct iterations, but it never had a variable-pitch stator. I suspect you’re thinking of the two-speed automatic introduced on the A-body intermediates for 1964, which is most commonly known as Super Turbine (or ST) 300, Buick’s name for it. The versions used by Buick and Oldsmobile from 1964 to 1967 did have a two-position variable stator. (Pontiac used the ST-300 for the Tempest/Le Mans and Firebird, but without the switch-pitch converter.) The ST-300 was similar to the late Powerglide, but not identical; they were not the same transmission.

          1. Buick DynaFlow’s were probably the first AT to use a variable pitch stator in the torque converter..a few years previous to all the turbo variations of Hydramatics.

          2. Yes, those are discussed in some detail in the Dynaflow article. However, I think the confusion in this case was conflating the two-speed Super Turbine 300 with the admittedly very similar Powerglide.

            Buick engineers seem to have been much more taken with the variable-pitch stator concept, probably because they spent so much time trying to create a truly stepless automatic transmission. Something I’ve often wondered about is how that fell chronologically with the use of variable-pitch stators in jet engines (including some jet engines Buick Division manufactured in the fifties), which ended up being quite extensive. I’m still not sure which application came first or if one inspired the other.

          3. question, i got a hydro-matic out of a 80-85 chevrolet delievery van, got it installed and driving off and on for 2 months, it shifts to second around 20 mph, fluid level good , all hoses intact. is that normal? ive drove for 30 mins to hour trip, hasnt overheated. any help would be appreciated. nathan

          4. I’m sorry, this is an automotive history website — I’m not able or qualified to provide repair or troubleshooting advice. Also, this article is about the original Hydra-Matic transmission, which has very little in common with the one in early eighties GM products other than the name!

  6. I would like to know the series numbers of the tranmissions that were built at the Hydromatic Plant in Three Rivers, MI. Ther years should be between 1979 to around 1994. This is when General Motors owned it.

    1. I’m afraid I don’t have that kind of technical information. The best I can suggest is contacting the GM historical archives and seeing if they have those records. Sorry!

  7. Hello – This morning I found your nice site while attempting to establish the invention and first use of synchonized shift transmissions, thanks for the great read.

    I want to draw your attention to the GM Automatic Safety Transmission for automobiles of 1937-39, which I had understood was the first commercial application of a practical automatic transmission by any maker. This transmission was invented and the prototype(s) made by Oscar Banker in his garage shop in northern Ohio, and subsequently pitched to GM. Although at once interested, by that date GM had invested heavily in H-M development and proceeded to the H-M transmission you outlined (read:’not originated here’ syndrome). Banker’s transmission was a much simpler design, which was offered essentially unchanged as the AST, and he also originated the safety shift pattern we now know (which H-M never had) and lobbied tirelessly as a ASE member for this shift pattern moving reverse range off the bottom in the name of safety. Banker’s transmission design evolved (slightly) in to GM V-Drive used in busses, etc. for decades. His and the transmission’s story is in a well written out of print biography of Banker by Robert ‘Bob’ Hull, and outlines this story in detail with footnotes, including theft of design lawsuits and what followed. I am also a fan of the H-M transmission, but this great story of one inventor getting crushed under (like Ford and Ferguson) rates a mention as his design and use of it passed the test of time – Best & Thanks

    1. I have read Banker's account and most of the underlying patents and addressed this issue on p. 2 of the article. However, I want to note the following:
      – Oscar Banker's transmission designs, as represented in his patent filings between 1927 and 1937, actually encompass at least FOUR substantially different designs with different layouts and different types of gearing. So, when one talks about “Banker's transmissions,” it's important to specify which one. (I assume the one he showed to GM in 1930 would have been the early dual-planetary variety.)
      – None of Banker's patents is identical or, at least from the layman's viewpoint, substantially similar to the GM designs.
      – When I initially glanced at Banker's 2,262,747 patent, I though it was surprisingly similar to the Automatic Safety Transmissions in some respects, but examining it and Thompson's 1934-1935 patents makes it clear that they're actually quite a bit different in both how they're laid out and how they work.
      – Even Banker did not claim that the Automatic Safety Transmission was his design, although he did allege that Hydra-Matic was an infringement of his patents (without specifying in what ways or aspects).
      – Some of the aspects that were similar between Banker's designs and GM's were not original to either party. Planetary transmissions, for instance, had been used in automobiles for more than 20 years at that point and several of the preselector transmissions had used multiple planetary gearsets to provide several different ratios. Banker certainly had no claim to the idea of an automatic transmission, designs for which were patented when he was only 9 years old.

      I am certainly not an attorney, much less a patent attorney, but studying the history of any kind of technology makes it pretty clear that patent cases are seldom as cut and dried as “bad actor steals innocent inventor's invention.” In situations like this, where you have a host of different, separately patented ideas and a mountain of prior art, it more often comes down to making a legal argument that certain specific design elements are similar enough to constitute infringement. That line typically has to drawn by a court and even then the verdicts are often appealed until one side or the other decides to cut their losses or seek an out-of-court settlement.

      So, no, the Automatic Safety Transmission was not Banker's transmission with a new name.

      Also, by Banker's own account, the bus transmissions Banker designed, which Yellow Coach did use, were produced in very small numbers. Banker said some of the buses using those transmissions remained in service for decades (which isn't difficult to believe), NOT that the design that replaced them — which Banker considered inferior — was a variation of his transmission. Building a few devices that remain in service for 30 years is categorically different than creating a device that remains in *production* for decades.

  8. I was surprised to read, “Deliveries to non-GM users didn’t resume until later in the 1954 model year,” because I’ve read elsewhere that GM took the noble step of delivering the first Willow Run Hydra-Matics to Hudson, Kaiser and Nash when the first units rolled off the line in October 1953.

    However, I’ll also concede that your article cites its sources, which is something that can’t be said for the other articles stating that such a gallant gesture was carried out by GM.

    1. You’re right, I’ve read that, as well, although it slipped my mind.

      The 1954 part I believe is correct — although it’s important to emphasize that it was the 1954 model year, not the ’54 calendar year. By the time the Willow Run plant had been converted, I think all H-M users (GM or not) were at least a few weeks into ’54 production. Even if GM sent the first units off the line to its outside customers (which they may well have done, as much for contractual reasons as magnanimity), early ’54 models would already have been built with alternative automatics. However, the “to non-GM users” part is possibly inaccurate, or at least misleading, so I’ve deleted it from the text.


      1. the 1953 pontiac production ended on november 20th,1953, quite some time before that date, pontiac was receiving regular shipment of the dual range hydra-matic from the new hydra-matic plant, during the time after the fire, a little over 17,000 pontiacs were built with the powerglide transmission, the 1954 pontiacs started production in december of 1953, and ALL 1954 pontiacs ordered with automatic transmissions had the dual range hydra-matics, the same goes for cadillacs and oldsmobiles. General Motors told rolls royce that they would stop suppling rolls with the dual range hydra-matics in 1966, not 1978 like you wrote. GM had rolls adapt to the buick version of the turbo hydramatic 400 transmission. you should know that the dual range hydra-matic trans was B&M high performance transmission choice, handling alot more horsepower after B&M modifications. the dual range hydra-matic was replaced by GM with the controlled coupling hydra-matic because the public wanted a smoother shifting transmission, not because of rising horsepower ratings from cadillac, oldsmobile, and pontiac engines.

        charles coker
        1953 pontiac technical advisor

        1. Charles,

          Thanks for the comment. Rolls-Royce did indeed switch to the TH400 for most of its cars around 1966, but the big Phantom VI limousine retained the old four-speed until 1978, finally switching to the Turbo Hydra-Matic when the 6,750 cc engine was adopted the following year.

          1. hi aaron, i don’t know where you got your information, but when general motors told rolls royce in 1966, that GM wanted rolls to adapt to the newer turbo hydramatic 400 transmission, that meant for all rolls royce models, not just most models. if it wasn’t profitable to supply dual range hydra-matics for all rolls royce models, it certainly wasn’t proitable to supply for one rolls royce model. i have search the internet, and can find nowhere any search results that backs up your claim of 1978. i know a rolls royce transmission rebuilder in the west los angeles, calif. area, 1966 was the last year for any rolls royce model using a dual range hydra-matic transmission.

          2. Charles,

            I don’t claim to be a Rolls-Royce expert, but if you don’t believe me, I would suggest looking up the Phantom VI in any number of Rolls-specific websites or something like the old World Cars volumes released annually by the Italian Auto Club in the seventies and early eighties.

            The Phantom VI was a formal limousine produced in very, very limited numbers for VIPs like members of the British royal family. The model wasn’t included in most price lists and I think it was available only on a special-order basis. As with the old Toyota Century and Nissan President or the Daimler DS420, which did similar duty, the Phantom VI was almost exclusively chauffeur-driven and then mostly for full-dress affairs. Between that and the extremely limited volume, the Phantom tended to lag well behind other contemporary models in technology. For example, the Phantom VI also retained drum brakes and the older 6,230cc engine long after most other Rolls-Royce models adopted four-wheel discs and the bigger 6,750cc V-8.

            I suspect that with the Phantom VI, there is an important distinction to be drawn between when components were *supplied* and when they were *used*. My assumption is that Rolls-Royce had a stockpile of the older Dual-Range transmissions — which the company would also have needed for service replacements and the like — and just kept using them in the Phantom VI until the supply ran out. Keep in mind that total lifetime production of the Phantom VI was in the hundreds, and that was spread out over a period of more than 20 years!

        2. My father was part of the team that perfected the automatic transmission. He was in GM’s experimental engineering group from 1937 to 1948. He was also chosen to be the driver of the Olsmobile that received the first working automatic after he designed the passing gear fluid system. This test made Michigan news paper headlines when they drove it across the state “first production car without a clutch peddle”. My father left GM and opened the first automatic transmission shop in the US. This shop was located in Phoenix Az, from here he supplied rebuilt HM transmissions all over the western half of the US. One of my fathers passions was horsepower and the method to transfer it the pavement. When you see the picture of a B&M Hydramatic opened up with all the modifications you are looking at my fathers transmissions/inventions, B&M bought all thier racing Hydro’s from my father, the only thing B&M ever produced was the B&M sticker they put on it.

          1. Did you father work with Earl Thompson?

          2. Any idea where I can find information on the old B&M hydra-matic modifications? I just bought a 1954 Starchief with a V-8 389 hooked up to a 1955 Slant pan Hydra-matic. I’m looking at what options I might have or recommendations on maintenance and the best ATF to run in it. Thanks.

          3. I can’t help with that, but perhaps someone else can offer some suggestions. Good luck!

    2. Regarding when GM sold Hydramatics to other car builders: My uncle bought a new ’51 Kaiser with an HM. I am quite sure they sold both to Studebaker and Nash about the same time.

      My father-in-law drove a 1953 Cadillar, which he claimed would regularly return over 20 mpg on the highway at 55-60. I don’t doubt that number.

      I’d heard from what I thouht were good sources that the net slippage was in the 3% range. I’m 80, and have had direct experience with most of the early automatics and semi-autos from Chrysler. As well as servicing some of them.

      Currently I am just finishing a novel, with as much good history as I can find, about how the small six car companies could have/should have been saved.

      1. The question wasn’t when GM originally started selling Hydra-Matic to other carmakers, but when they *resumed* deliveries after the fire destroyed the original Hydra-Matic plant in Livonia in 1953. As the text indicates, GM certainly sold transmissions to outside companies before then, but the destruction of the plant obviously interrupted deliveries both to GM divisions and outside customers.

      2. Net slippage in fourth gear was 3% or less, but it’s important to note that the way the fluid coupling is arranged relative to the planetary gearing means that you have significantly different rates of slippage in each forward gear, which was an intentional design choice. It’s really very clever, although it takes a while to get your head around.

      3. Not to Studebaker: they developed automatic drive 3-speeds with a lockup 3rd gear for the 1950 model year and later bought non-lockup 3 speeds from Borg Warner from about 1955.
        Kaiser, Jeep, Hudson, Lincoln, Rolls Royce, Nash, and GMC did source 4 speed HydraMatic.

        1. You may be interested to know that our next article will deal in some detail about the workings of the Studebaker/Borg-Warner DG.

        2. Actually, Studebaker used a 3-speed automatic transmissions with a lock-up torque converter. It was developed and manufactured by Borg Warner and introduced in 1951. I have driven Studebakers with that transmission. Some years after 1951 they deleted to lock up clutch on the torque converter.

          Rolls Royce, under license from GM, basically copied the GM 4-speed Hydramatic.

          In I believe 1955, Lincoln dropped the GM Hydramatic and used a Borg Warner 3-speed automatic.

          1. Incidentally, the early Studebaker Automatic Drive/Borg-Warner DG is discussed in some detail in my article Giving Slip the Slip.

            If I were going to be really pedantic, I would describe the DG as having a direct-drive clutch rather than a lockup torque converter. Typically, a lockup torque converter allows the torus cover or flywheel/flex plate to drive the transmission input shaft, so the converter lockup and the actual transmission gear are independent of one another. In the DG, the high clutch allows the flywheel to drive the output shaft, so direct drive is a completely mechanical connection. It doesn’t allow converter action in third gear, nor does it permit the converter to lock up in any other gear.

      4. GM sold HydraMatic to Hudson, Nash, Lincoln, Kaiser, Rolls-Royce, and GMC. Studebaker developed it’s own Automatic Drive 3 speed, and Packard it’s own Ultramarine independantly.

      5. Your last line: does it refer to an input from me a time back? It is exactly what I have been working on for four years!!! Every time I sit down to do some research, it seems I find a new bit of info I need to address. Hopefully, I will be able to publish soon. All 300 plus pages. Lots of car industry info, as well as cars, components, coulda/shoulda beens, etc.

  9. hello, i find some of your statements about the hydra-matic in error. the fluid coupling does not waste fuel or slips alot at low speeds, my own 1953 pontiac chieftain custom catalina would always get 15 to 17 mpg city, and 19 to 20 mpg hwy. the fluid coupling barely slips enough at idle to allow the pontiac straight eight to keep running with a in gear idle speed of 365 rpm’s. the hydra-matic’s low slippage equal ideal transmission fluid temps that only heavy duty cars like taxi’s and police cars needed a fluid cooler installed. as far as handling more than 165 horsepower, you only have to look at what transmission made B&M transmissions famous. the two best automatic transmissions to ever come out of detroit is the the 1955-56 dual-range hydra-matic, and the 1964-67 variable pitch turbo-hydramatic 400. charles l. coker, 1953 pontiac tech advisor, poci.

    1. Mr. Coker,

      Thanks for your comment. The remark to which you’re referring was a general description of the nature of fluid couplings, not specific to the Hydra-Matic. To function as a clutch, a fluid coupling has to have enough slip to absorb the engine’s idle torque, otherwise the engine will stall if the brake is engaged. The amount of slippage at engine idle speed or just off idle is therefore a lot higher than the slippage at cruising speed — more than 90% versus less than 5%.

      That said, the Hydra-Matic’s fluid coupling had considerably less slip than most, if not all, of its early rivals, and as you note, HM-equipped cars could return quite good fuel economy. Some of that was achieved by using a lower numerical axle ratio than manual-shift cars, but the Hydra-Matic was certainly far more efficient than the early Dynaflow, Powerglide, or Ultramatic. I imagine that if one did a proving grounds comparison between a Hydra-Matic car and a manual-shift car (without overdrive) with the same axle ratios and tire sizes, the manual shift car would probably have a slight edge, but in the real world, it would likely be a wash.

      1. Not to Studebaker: they developed automatic drive 3-speeds with a lockup 3rd gear for the 1950 model year and later bought non-lockup 3 speeds from Borg Warner from about 1955.
        Kaiser, Jeep, Hudson, Lincoln, Rolls Royce, Nash, and GMC did source 4 speed HydraMatic.

        1. Did you mean this in response to Dean’s comment? You’d replied this back in August, which you can see elsewhere in this thread.

          But yes, in regard to this comment specifically, the Studebaker Automatic Drive/Borg-Warner DG (which is discussed in more detail in the split torque article) had the edge in cruising efficiency, since its converter is completely ineffective in third gear, whereas Hydra-Matic has, in effect, a partial lockup in third and fourth gears.

          1. Actually, the Hydramatic had far less slip in 2nd gear than in 1st gear. I’ve driven 1953 Pontiacs with Hydramatic and a 1953 Oldsmobile with Hydramatic.

            “L” on the selector was really 2nd; you could actually start in 2nd. With the car standing still and the brakes on, if one shifted from either drive position to 2nd the engine would noticeably slow down and, on the Pontiac, almost feel as if it was lugging. When accelerating from a stop in 2nd the engine revved very little at first and the car felt sluggish partly because the minimal slippage would not permit the engine to develop much torque.

            In first gear the power flow was through the front planetary, then the fluid clutch, then the second planetary. That was why there was more slip in 1st gear than in 2nd gear.

          2. Yes, the object of the Hydra-Matic’s unusual layout was to have fairly steady reductions in slip as you moved from first through fourth. If you want to be technical about it, the power flow is always through the front planetary gearset in the forward gears because the impeller of the fluid coupling is driven by the front gearset’s planet carrier rather than by flywheel. However, in first (and third), the front gearset is in reduction, so the front gearset drives the impeller at 69 percent of engine speed with 1.44 times engine torque. With the shift to second, the front clutch engages so the engine is now driving the impeller in direct drive, without the additional mechanical advantage; that’s probably why the engine loads up when you shift to “L.”

            In terms of the performance difference between first- and second-gear starts, the substantial difference in overall ratio is probably more significant than the difference in hydraulic slippage. With a ’53 Dual-Range Hydra-Matic, first gear is 3.82:1. In second, you’re running only on the rear gearset, which has a ratio of 2.53:1. Since Hydra-Matic cars typically also have a taller axle ratio, it’s no wonder they end up feeling sluggish, especially with the old Pontiac straight six and straight eight, which weren’t exactly torque monsters to begin with. That’s about the same ratio as first gear on a three-speed manual car, but with the taller axle ratio, the coupling slippage, and the added load of the transmission oil pumps, the automatic car was obviously at a disadvantage.

  10. Thanks for the great story and information. My late mother, who began driving before WWII, boasted that she never learned to drive a car with a manual transmission. She learned to drive on her mother’s 1937 Oldsmobile with AST, which my grandfather bought from an Oldsmobile rep traveling through the South. My mother later drove the car throughout the WWII years. She said that other than using the clutch pedal to change the gear quadrants, she left it in “High” and that it shifted very much like Hydra-Matic, which she drove for many years afterward. And speaking of Hydra-Matic, I remember as a boy back in the 1950’s, the women in the family talking about the Hydra-Matic “jump”. When starting out, they would always make sure that the gear selector was in the correct quadrant (especially when choosing “Reverse” on the far right side), give it the gas, and then release the parking break. I also remember Hydra-Matic having a rather unique and pleasing sound that is not heard in cars today.

  11. Two points concerning the fluid coupling in 1940 to 1955 HydraMatic transmissions:
    1. In first gear, the flouid coupling is driven through the front planetary gearset. Thus, the coupling is driven at about 3/4 of engine speed. This reduces “creep” and allows the engine to reach a higher RPM at takeoff.
    2. When the transimssion is in fourth gear, the fluid coupling is effectively locked out and there is no slippage at all. The original HydraMatic was an engineering tour de force, a brilliant design with many patents that frustrated competitors for years.

    1. Mr. Andrews,

      Some of these issues were unclear in the previous version, which since I've taken pains to rectify. However, to your points:
      1) The impeller is always driven the through the front planetary gearset, but it's only through the front planet gears (and thus in reduction) if the front brake band is engaged, which means in first, third, and reverse. (In second and fourth, the front clutch is engaged, so the annulus and planet carrier are locked together in direct drive.) The purpose is as you say: to minimize creep in first or reverse with the throttle closed.
      2) The coupling is never actually locked out completely. In third and fourth, torque is split between the coupling and the rear planetary gearset, but the intermediate shaft turns both the impeller and the rear planetary gearset at the same speed, so some power still goes through the coupling. Locking out the fluid coupling entirely would have been a neat trick, but the only provision for doing that is in Neutral.

      It is a clever piece of work, there's no doubt about that.

  12. The first car I ever drove was a 1958 Pontiac with Hydramatic – still, after all these years, the best Automatic ever made. Unlike modern computer controlled automatics, the H-M was analog – shifts were controlled by speed, oil pressure, and many other factors too technical for my level of expertise. In short, they made H_M seem as if it had a personality; indeed, each one I drove (Dual Fluid Couplings from 1956 on)had a distinct feel. Can’t prove it, but my feeling is that GM paid more attention to the smoothness of the shifts the more expensive the model. My friend’s parent’s ’59 Olds 98 Convertible had a 2d to 3d shift that you couldn’t feel, only hear the revs drop. My Pontiac, on the other hand, had a very pronounced 2 to 3 shift. And yes, echoing an earlier comment, they had a distinctive sound.

    1. I’m not sure exactly how much difference there was in the dual-coupling Hydra-Matic across divisions. With some of the other transmissions, there was a lot; Turboglide and Flight Pitch Dynaflow, for example, were different transmissions, even though they were each based on the same Engineering Staff concept. On the other hand, those transmissions were manufactured by the divisions themselves, whereas the Hydra-Matic came from Hydra-Matic Division. I would assume that there were at least minor adjustments to tune the transmission for each division’s engines — the Oldsmobile, Pontiac, and Cadillac V8s in those days were quite different, and didn’t have the same torque curves — but I don’t know how extensive those changes may have been, or if there were also significant variations in internal materials, transmission mount design, etc.

      Complicating that question is the fact that there were a lot of minor revisions to the dual-coupling Hydra-Matic during its first few years, to improve reliability, deal with teething problems, and so forth. Furthermore, the dual-coupling transmission was a very, very complex bit of business (part of the reason GM eventually phased it out). Minor production variations between individual transmissions might also make for small but perceptible differences in feel.

      Of course, it’s not improbable that someone at Pontiac told Hydra-Matic Division, “Hey, we’d like a little firmer shift quality, for a sportier feel.” On the other hand, it’s also possible that the transmission in a ’59 Pontiac might feel a little different than one in a ’58, especially if they were at opposite ends of the allowable production tolerances. Hard to say.

  13. Hello!
    In first place i´d like to congratulate your work here. Fantastic! Very informative and entertaining. Now about the topic, and just to measure the historical importance of the first Hydra-Matic, here in Brazil people often call any kind of automatic transmission “hidramático” (it´s how they called them in the late ’40s, early ’50s, and the name is still in use!). But the “hidramáticos” still are kind of rare in our country, people prefer to change the gears by themselves. Only in the 2000s automatic transmissions started to take off in sales, and in 2010 they represented 16% of total sales (compared to just 5.5% in 2005).
    Now a little off topic: You mentioned the Willow Run Manufacturing Plant, was that plant constructed by Ford Motor Company for the construction of aircrafts?
    Thank you for your articles and information!
    Best regards

    1. Gabriel,

      Yes, the Willow Run plant was originally constructed by Ford, building Consolidated B-24 Liberator heavy bombers under license. At the end of the war, it was leased by the new Kaiser-Frazer company, which later bought the plant outright. Kaiser sold Willow Run to GM in 1953.

  14. Sir:
    I am curious as to what oil is used the original Hydra-Matic trans. The military used the Hydra-Matic in the GMC (1950-1955) and used Heavy-duty engine oil as a hydra fluid.
    Can a SAE 10W High-detergent work in this trans?
    Marty Meaney
    Ludlow, VT

    1. Marty,

      I’m afraid I’m a historian, not a mechanic, so I’m really not qualified to give technical advice. I’d suggest talking to a shop that overhauls early Hydra-Matics and see what they recommend.

    2. Oldsmobile in the early days advised that 10W motor oil could be used in an emergency. I bought a 55 Olds 88 in 1978. It had 46,000 miles on it. The “original” transmission fluid did not have the red dye we associate with ATF or Dexron. I taught for Allison Transmission locally from 1978 to 2003. They reccomend Dexron for all automatics but for “off Highway” use you can use “C” approved fluids which are motor oils that have been approved by Allison

    3. the answer is type A, which is still available today, although most original hydra-matic owners today are using dexron with good satifaction.

    4. I am involved part-time with forest fire suppression and in that capacity have driven several different early fifties GMC 6X6s converted for hauling water. Most have 1,000 to 2,000 Imp. gallon water tanks installed. They all had the GM dual-range automatic transmission. They have got to be the roughest shifting automatics I have ever had the displeasure to drive, crashing into gear with no way to control the shift. With shift kits installed that turned them into manually shifted automatics most operators could learn to rev match the shift for a good deal smoother operation. But the factory transmission is no fun to drive.

      1. Setting aside any maintenance issues, I would guess that Dual-Range Hydra-Matics built for heavy-duty applications shifted more harshly than ones intended for passenger cars. Heavy-duty clutch packs and a hydraulic valve body set up to take a lot of punishment aren’t known for smoothness in general; conversely, slow, slurred, gentle shifts are easier on passengers, but not good for transmission longevity under heavy loads.

    5. the original fluid is type A, if you had to, in a pinch, you could use 20w oil, then change as soon as possible.

  15. Can anyone here tell me how many of these 3100 pickups came with the hydromatic? I have the serial# and also the Detroit Transmission Division #off the side of tranny.Please contact me if you can help, and I will give the #s’ Thanks, Rod

    1. I honestly don’t know. The figures GM supplied for Hydra-Matic production unfortunately did not include the truck units, and I have not seen totals elsewhere.

  16. i have a hdra-matic that has the b&m automotive chevy bolt pattern bellhousing on it that is a very interresting transmission love to hear about the history on that transmission it is hard to get information on old transmission

    1. Might be a Hydro stick B and M transmission that was made in the 1960s by B and M. It was one of the first drag race automatics.

  17. Hi, I was tring to find some information, on the 1938 Oldsmobile AST transmission. I cannot understand why the information is so hard to find? I owned a 1938 Olds with the AST and a 6 cylinder engine. I did not like it and let my brother use it. He complained that, it cost more to put oil in the transmission, than it did gas. They used motor oil in the trany. So I sold it.
    My first boss had a 1937 Hudson, with there Electric vacumm shift transmission, which I drove, at times. Both of these early tranys are hard to find info on.
    Art Baethke

    1. Art,

      The AST was short-lived, and really quite rare. As best I can figure, production was no more than about 25,000 units, and as you saw, customer reaction to it was not great. I suspect that there weren’t too many AST-equipped left on the road by the end of the war. Also, I don’t think Oldsmobile encouraged dealers to take them apart; there was a program that let people trade a defective unit for a replacement for about $75. So, I’m not sure if there was even a dealer repair manual for it.

      The Hudson system is a little better documented. (It was made by Bendix, known as the “Electric Hand.”) It was sort of an add-on vacuum/electric preselector system — it added a vacuum cylinder to the gearbox that would execute the actual shifts. There was a selector switch on the steering column that you would use to choose what gear you wanted, and another (usually attached to the clutch pedal) that would trigger the actual shift via solenoid. You could also order Electric Hand with a vacuum-operated clutch that would disengage automatically if you lifted your foot completely off the throttle and then reengage when you pressed the accelerator. (Hudson promoted the combination as Selective Automatic Shift, but it wasn’t exactly automatic, and as far as I know, the preselector and the vacuum clutch were technically separate options.) Hudson-Terraplane clubs would probably know where you could find service manuals or other technical info, if you wanted a more detailed explanation. Likewise, Cord collectors are familiar with it, as the Electric Hand was used on the 810 and 812.

      1. I had a ’36 Terraplane (built by Hudson) with the magic hand. All of these cars were delivered with a “stick” that was under the front seat. It plugged into the transmission, just like any floor shifter of the era. That was the only way to shift the one I had. The standard joke was that the magic hand was gauranteed clear to the end of the dealers driveway, or until the first time you used it.

  18. I remember back in about 1959 we had a 58 Pontiac station wagon that ran real good. But one time our family went to stay a weekend on a farm out in Kansas. It had rained the night before we returned and the dirt road turned to mud. Well that Pontiac really plowed through the mud ok in low, but a week later the transmission went out. The Pontiac dealer said the transmission had burnt up and needed a rebuild. That car had several transmission rebuilds before it was traded in on a 63 Pontiac. The transmission was kind of weak as I remember, even though it shifted just fine. I think it had two ranges on the shifter. Today you would use a FWD truck for that type of use, but back then it was common to use a car like a truck.

    1. The 1958 Pontiac would have had the second-generation Hydra-Matic, which Pontiac called Jetaway. It was similar to the original Hydra-Matic in some respects and quite different than others; it was intended to provide smoother shifts than the original. There’s more about that transmission in the second part of this story.

  19. The Hydromatic was used in 1950 and 1951 Lincolns with the 337 Cu inch flathead V8.
    The Ford o matic I guess could not cut it behind the big 337. Hydromatics were used in Lincolns up to about 56 or 57. After that, a big Fordomatic design was used. I drove a 1950 Lincoln once and the transmission worked ok.

    1. The Hydra-Matic was used in Lincolns from 1950 through 1954. In 1955, it was replaced by Turbo-Drive, a three-speed/torque converter automatic. I’m not sure if it was a bigger Fordomatic in concept and design; I’ve always assumed so, but I haven’t looked into it in any detail.

      1. Rolls Royce used a Hydromatic for a while. Some where I seen a list of transmissions that were used is different cars back then. I drove a military two and a half ton truck once that had a Hydromatic. What a slow truck it was. I thought I was going to have to get out and push a few times. It was a real big transmission as I recall, but it looked just like a Hydromatic for a car. Had a real cool looking gear shift on a stand. No one wanted to drive it, but I did not mind if you was not in a hurry to get some where.

        1. Rolls-Royce built the Dual-Range Hydra-Matic under license for many years. In the mid-sixties, it was phased out of Rolls-Royce and Bentley passenger cars in favor of the Turbo Hydramatic (purchased from GM’s Hydra-Matic division), but Rolls continued using it in the big Phantom limousines until 1977 or 1978. I don’t know how long they used it in military vehicles or trucks.

  20. I have a 1940 oldsmobile 6 cylinder can you tell me what engine size it has its a hydromatic.

    1. Manuel — The 1940 Oldsmobile six was 230 cubic inches (3,764 cc). It had 95 gross horsepower.

  21. The 1905 Sturtevant offered a patented “automatic” transmission.This device was a was a “three speed,consisting of a series of clutches,operated by centriugal force.”The make was high dollar($5,000)and was
    offered 1905,’06,and ’07 with the automatic.

    Push-button electric shifting made by Hammer-Cutler came out about 1913 and appeared in the SGV,Pullman,Haynes,etc.I believe it was called
    a Vulcan,and was a 4 speed.

    Around 1917 another expensive car came out with
    an electric transmission.The Owen-Magnetic had no gears to shift.

    1. I took a look at the Sturtevants’ patent for the device (U.S. Patent No. 766,551, issued 2 August 1904), which is an interesting piece of work. The patent describes a two-speed transmission, although it notes that the same principles could be used to add additional speeds by adding additional clutches. I wonder how well it actually worked.

      I’m not familiar with the Hammer-Cutler system or the Owen-Magnetic, but I’ll look into them — thanks!

      1. Hmm, there was also a continuously variable transmission invented by Frank Hayes that was offered briefly in 16 and 18 HP Austins from 1933. It appears to have been broadly similar to the modern roller-type CVT (such as Nissan’s Extroid unit), but it retained a manual clutch, which had to be used for stopping and starting.

        1. The Frank Hayes transmission may have been the basis of the Buick “Roller” transmission (which was apparently based on an outside design), but I'm still not certain about that.

          1. Here’s an interesting page on the Hayes transmission:

            www. austinmemories. com/ styled-23/ index.html

            I read somewhere that the transmission was troublesome, and Austin retrofitted their standard stick-shift transmission to most of the cars that had had the Hayes transmission.

            One of the British motor museums has a Hayes-equipped Austin, and they’ve somehow gotten the transmission to work, although I don’t imagine the car sees many miles.

          2. Probably not. I’m still not entirely sure if the Buick transmission was the Hayes design. (It may have been, but there are several contradictory accounts.) It’s an interesting idea, but it took many years of metallurgy to make roller-type CVTs practical.

  22. I’ve got car 74(1905 REO)and I forgot to mention that 1933 was the year REO offered the
    Self-Shifter.I think it came on either the
    straight eights or the sixes.It must have been
    functional as they were offered multiple years
    and there are several surviving cars with this

    I recently saw a 1938 Studebaker for sale with an electric hand Bendix Shifter.It was my
    understanding that it was a factory option.

    1. The Reo Self-Shifter was introduced in the spring of 1933; it was standard on eights, optional with the six. It apparently worked reasonably well, but it was not an automatic transmission in the modern sense — instead, it was a four-speed semi-automatic, analogous in concept (though not in mechanical layout) to the later GM Automatic Safety Transmission.

      One of the things that can be very confusing about this subject is that for many years, the terms “automatic” and “self-shifting” were applied rather loosely to all manner of transmissions, only a few of which we would consider automatic. Some were semi-automatic transmissions, like the Reo Self-Shifter or the AST, while others were preselector transmissions along the lines of the Cotal or the Bendix Electric Hand. Preselectors were not automatic except in the sense that the actual gear change was executed by some kind of remote mechanism, rather than through the direct movement of a shift lever or cable linkage. The driver still had to decide what gear to use and initiate the actual shifts. (I would draw an analogy to the way the term “automatic” is applied to pistols or rifles; it’s often used to describe weapons that are simply auto-loading, rather than truly automatic.)

      Like the AST, a lot of the semi-auto and preselector transmissions retained the clutch pedal (although in a preselector transmission, it was essentially converted to an engagement switch to execute the actual shift) and generally required at least some manual shifting. Few of them did well commercially, particularly during the Depression — buyers were reluctant to spend the extra money and risk the potential reliability hassles if they still had to change gears and operate a clutch pedal.

  23. GREAT GREAT ARTICLE…It helped me a lot

    I have a 1941 Cadillac Series 61 (Model 6109D). Is my shift pattern as shown above?
    Neutral, Drive, Low, and Reverse
    Do I need to pull in (or push out) the shifter to get it into any gear?

    Once again Thank you

  24. My Great Uncle, Kenneth E Snyder was an engineer at GM for 30 years. He worked there from approx 1936 to 1966. On the back of the watch given to him by GM states: Service Award 1961 Detroit Transmission Division GMC K.E. Snyder 25 years. My Great Aunt Marion, his widow, gave the watch to my son after my uncle died. Uncle Kenneth was on the team which developed the hydramatic, from all the stories that I’ve been told, plus, the last time I was at his home in FL , I saw the award GM gave to him which was a small trophy of a transmission. I wish I knew exactly what it said. Our family has always been very proud of his work at GM and always knew how smart and gifted a man he was. I’m sure the whole team knew they would never receive individual recognition, auto companies never worked that way. Anyway, I just thought all you car people would enjoy hearing my story. My brother, Kenny, lives in FL and really knew my Uncle MUCH better than I and would be a much better source of information.

  25. Hello! Certainly interesting historical information here. Thanks for the attention to detail. I was hoping that someone here might offer some advice on an HM unit that I’ve put in my 41 Cadillac 6227D coupe. The car was originally HM, rare for a coupe and I’ve just about completed a frame off restoration. I found an older gentleman who used to work for GM HM to do a rebuild of a ’48 HM unit (41’s are really not too good). He did it and I have pictures of each step of the rebuild. He’s now passed on. I have installed the unit, filled it properly, and adjusted the bands according to the 48 shop manual. The car moves in Hi, Lo, and R. R seems fine but the manual shift out of R is a little tough. [b]Lo has one speed, no shift and I’m not sure if that’s correct. In Hi, the car takes off, but misses the shift and feels as if it’s shifted to N. [/b]I’m hoping that an adjustment to the throttle control linkage may be the issue and that it’s not something internal. I’ve tried adjusting the bands 1/2 and 1 full turn tighter with no effect. There are no strange noises associated with the problem. Again, in Hi/D upon acceleration the car gets up to about 10mph and then feels as though it shifts to neutral rather than up shifting to 2nd.
    Any help would be appreciated.
    Ken Karrer 1941 Cadillac 6227D coupe

    1. Ken,

      I’m not qualified to perform troubleshooting or repair advice, but to the best of my knowledge, the early Hydra-Matic IS supposed to shift between first and second in Lo range (comparable to the “2” position on later automatics). I would suggest digging up a shop manual for the ’48. You might even find at the library — I was able to find and check one out while researching this article.

    2. Ken

      It’s been a while since your post, so you probably got your questions answered … but if you’re still having issues here’s a few comments.

      I agree with Aaron, don’t want to try to diagnose your problem here … but I can’t help making a couple quick comments …

      I’m an Olds guy … so I did a little quick research on your questions since Olds, of course, used the same Hydra-Matic as Cadillac (or mostly the same as far as I know). We had several Oldsmobiles with Hydra-Matic back when I was in high school, that I used to work on. And I have always been interested in transmissions (I’m an engineer by training, though I don’t work in engineering).

      It sounds like you have two issues/questions:

      1) What is correct operation in “Low” range?
      2) Bad 2nd to 3rd shift (goes into neutral)

      Item #1 – In Low range the trans should start in first and shift to second, but at a somewhat higher speed than in Drive range. If shifting into Low range at cruising speed, the downshift sequence gets a little more complicated, but that’s another discussion.

      Item #2 – The 2nd to 3rd Hydra-Matic shift is quite complicated due to the required timing of the apply and release of the friction elements. The 2-3 shift is probably the major contributor to the Hydra-Matic’s reputation as having harsh shifts.

      In second the front planetary is locked up with the clutch applied, the rear planetary is in reduction with the rear band on. In third this is reversed: front planetary goes into reduction by releasing the front clutch and applying the front back; the rear planetary up shifts by releasing the rear band, applying the rear clutch. If the timing of these “release” and “applies” is off then one gets 2-1-3 (front planetary downshifts faster than rear planetary upshifts) or one can get 2-4-3 (rear planetary upshifts before front planetary downshifts) … or 2-neutral-3 if either the front band apply is too slow (after the front clutch released) or the rear clutch apply is behind the rear band release. As I think about it, mis-adjusted bands seems like the most likely cause of a 2-neutral-3 shift, though it isn’t explicitly listed in the Diagnosis section of the Service Manuals I looked at. If you have a mis-adjusted band (singular) you should be able to figure out which by observing other shifts, but not necessarily.

      Despite having adjusted the bands once, I’d recheck adjustment.

      If bands are ok, next stop would be looking for sticking valve in valve body.

      If you’re still having problems .. post something here and we can communicate outside this forum.


  26. This article is fascinating to me. At my current age–66–I drive a Grand Marquis LS 4 door sedan. What I would give for a dual range hydramatic drive transmission. Before starting at OSU in Stillwater, OK, I worked at Edward’s AFB and purchased a fast back 1948 Cadillac with hydramatic drive. I wish I had never parted with that car! Thanks to the authors of this engrossing article/history!

  27. This article contains much useful information about the Hydramatic. Earl Madman Muntz put the Hydramatic in the Muntz Jets from 1950-1953. A project Jet I’m working on has a 337 cui Lncoln flathead mated to a 1952 Hydramatic. Take 8 qts. of Type A fluid througha funnel in the dipstick hole. Hope to get it running. Right now it seems to have mineral oil in it. I guess it was used back in the day.

  28. Hi,
    first of all thanks for the nice reading, it’s the first time i see this site, i’ve run into it casually!

    "The best I can suggest is contacting the GM historical archives and seeing if they have those records."

    I’m afraid they don’t have that information, I already checked.


  29. I really enjoyed the story and have a question about the production facilities. GM built the Livonia facility in 1948 according to the Livonia history website. I’m curious as to where were they produced from 1939 until 1948? Was there a dedicated factory?

    1. You know, I don’t know. That’s a very good question.

      1. Ah-ha! Finally something I can contribute to! lol. I was actually searching for info on the Detroit Transmission Division of GM when I stumbled on this page. Except for the real technical stuff, I actually found it very interesting! Especially the GM history. (my dad retired after 42 yrs at GM and my mom got an early retirement at 27 yrs so we lived and breathed GM. lol)
        Anyway, the plant you are asking about, where the transmissions were produced starting in 1939? That plant is the reason I’m scouring the internet on a sunny Saturday in Detroit. The first building GM built the Hydra-Matic transmissions was located on the corner of Riopelle and Farnsworth in Detroit. It was near Warren and I-75, on the east side of Detroit. The building started out sometime around 1917 as Fisher Body Plant #10, where presumably, Fisher made car bodies for GM and many other car companies at that time. (until a deal was struck in 1926 for them to become the in-house body maker for GM, according to Wikipedia) It’s a little unclear to me if they continued to make bodies for GM there until 1939 when the Detroit Transmission Division was formed and housed there or if there were other operations in between.
        The H-M was built here until the new facility in Livonia was ready in 1949. We all know what happened within a few years of that. Below is a link to a brochure on the GM Heritage Center’s website, showing a timeline history of the Detroit Transmission Division, with the first line saying that the division was organized “in one-third of an old six story building on Riopelle Street in Detroit”.

        [www.gmheritagecenter. com/docs…elcomesYou.pdf]

        Although I don’t see a date this was printed, it was after 1957 since they list that year on the timeline but I’m sure it was before 1960. But what I find funny is, they write off the original building as “old” when, in 1939, it was just barely over 20 years old and even if it was 1959 when they printed this brochure, it was just barely over 40 years old! Now, this was an Albert Kahn building, top of the line, re-enforced concrete that was made to stand the test of time. But, because it wasn’t the “new hotness” like the Ypsi plant they’re advertising in the brochure, it was just an old building. I’m surprised they even named the street it was on. Anyway…
        After Detroit Transmission moved to Livonia, eventually GM moved some Cadillac production in there for quite a few years and then used it as a warehouse until the early 80’s. And GM people know what happened then; plants started closing left and right. My parents were both working at a plant that closed in 1987, I believe, and I think that one lasted longer than some others. (Luckily my parents both ended up at the Warren Tech Center and were able to leave GM on their terms, not GM’s.)
        It’s a little fuzzy at this point, as far as this plant but I know a food wholesaler was in there for a while in the 90’s – Total Foods, until the people I work for bought it in 1998. They used it as a rental property (it was 500,000 sq ft) but used a big chunk of the space as a warehouse, shipping facility and eventually an assembly operation was added, for the manufacturing company they also own, Palmer Distrib. This is the company I actually work for but I am basically the admin asst for both of the owners, but in their main office in St Clair Shores. I never worked in the warehouse.
        This building did stand the test of time, for a total of just under 100 years. I’m sure it would have lasted a lot longer too but a nasty, nasty fire took it on February 5th, 2014. It started on the 3rd floor and swept thru the building like it was a house of cards. It burned for 5 days, smoldered for 2 days and then burned for 2 more days. Someone close to the Detroit Fire Dept put together the video at the link below, if you’re interested in seeing the first couple days in 7.5 minutes: [youtu. be/wKe63ZdRsec]

        That happened at the beginning of February and the last part of what was left of the building was knocked down at the end of May. I guess they’re still cleaning up the rubble and rest of the property. I think I heard mid-July is when they expect to be done with that.
        So, now I’m helping someone do some research on the history of the building for a paper they’re writing. This is what brought me here. A lot of the early history I shared came from that other person’s research. I am obviously more familiar with the more recent uses of it. I have one more link to share, in case any one is still skeptical that this building I’m referring to is in fact part of the Detroit Transmission Division of GM….

        [www.emporis. com/ building/detroit-transmission-division- general-motors-factory-detroit-mi-usa]

        1. Christine,

          Thanks for the information! I will have to check this out.

          1. Aaron,
            I’m glad I could contribute to the conversation. My friend is supposed to be posting his paper about this building on his website within the next couple of days. I will post the link to it once it’s up. It should include pictures from after the fire but before the demo – inside and out.

  30. Thank you so much for all of your effort here, and with the article!
    In my years( 83-03) as a New Car Manager & U/C Mgr in a multi-point GM store, I guess I have driven everything G.M. ever produced domestically, say from 1970 and newer, and many a lot older. (just a guess) I never thought to bring this up to the Factory rep, who may have been able to comment on my question. MY mother has driven Cadillacs going back to 1966 through today all except one of them being Sedan Deville’s, the exception was an ’82 or ’83 Coupe powder gray beauty, I was not permitted to drive them, of course, but now at 53 she may allow me to gas it up for her.. Loves her Cadillacs!
    Why does a Cadillac tranny shift feel differently to me, it is just a unique feeling of smoothness & power? Now, that’s me. Have you ever heard this or experienced for yourself?

    1. Well, even where different division share the same basic transmission, each division’s engineers have to tune the transmission to suit the torque characteristics of the division’s engines and their cars’ identity. With planetary gearset automatics, the transmission’s performance depends a lot the timing and firmness of clutch engagements and so forth; slower, slurred shifts tend to be smoother but less efficient, while firm, fast shifts are the reverse.

      Finding the right balance for street cars is as much an art as a science because it tends to involve a lot of compromises. GM divisions tuned the Turbo Hydramatic (which most Cadillacs between 1965 and 1981 used) in various ways; some were very smooth, others produced back-slapping shifts (intentionally).

      Cadillac had two advantages during much of that time: first, their engines generally had lots of torque, and second, Cadillac customers weren’t looking to extract maximum quarter mile performance. As a result, their engineers could strike a more relaxed balance on shift quality without sacrificing too much performance. They could also more thoroughly isolate the transmission mounts from the car in ways that Supercar drivers would’ve complained about endlessly.

      1. I should add that even within the individual divisions, there were multiple variations of the Hydra-Matic for different models or applications. For example, in the early fifties, Cadillac had three or four variations. The ratios were the same. I assume differences were chiefly in the clutches and control valves to handle different load requirements, although toward the end of the single-coupling era, some applications also added an oil cooler.

      2. This is an interesting site; I just discovered it. It is loaded with good historical information. Obviously a lot of work and study has gone into it.

        Some decades ago, I rode in a 1953 Cadillac with Hydramatic. The shifts were VERY firm; it was impossible not to feel them. The owner, who had bought the car as part of an estate, stated that it was intended to have firm shifts. On the other hand, I remember riding in a 1950 Pontiac convertible. The shifts were so smooth that you absolutely could not feel them although you could hear the change in engine speed. The same was true with, I believe, a 1949 Lincoln, which a landlord owned.

        I’ve ridden in many cars made from 1950 through the early 1960s with Hydramatic. Even before the 1956 Jetaway fiasco, there was considerable difference in how they operated. I tend to favor somewhat firm shifts the theory being that firm shifts result in less wear and longer life of the friction surfaces.

        It would be interesting to take a 1955 Hydramatic, remove the hydraulic “brain”, and replace it with electronically controlled solenoids.

        1. The firmness of shifts in the early Hydra-Matic had a great deal to do with how precisely it was adjusted, which was something of an art. Compared to later transmissions like the TH400, there’s just a lot going on mechanically with each shift, so slight timing variations can really add up.

  31. I’m very pleased you acknowledge the part the English Daimler Company played in the birth of Hydramatic. What Kettering & co aimed to do was give the Daimler transmission a brain, and the hydraulic servo system, controlled according to speed and load, was that brain.
    But the basis remained the same – four speeds, fluid coupling and steering column lever control, so allowing three to sit abreast in front.
    The Rolls-Royce made hydramatic was licenced for 5,000 units per year, I believe, so they sold hydramatics to Jensen, Armstrong-Siddley and I think a couple of other posh low-volume British marques.

  32. I’ve enjoyed reading several articles on here today and they’re mostly very good, but I have to take some issue with parts of this – mostly detail and tone… possibly from a need to brutally economise to get it within 4 pages despite having an obvious wealth of information to include, IDK…!

    First up… yep, “sliding-gear” manual transmissions ARE desperately out of date, obsolete, irrelevant, etc… and indeed, they were so well before WW2. One of the later cars to continue using actual “crash” gears, with straight-cut gears moving around in relation to one another, was actually the Ford Model A with its 3-speed manual version (not all old Fords were 2-speed epicyclic!). Even before synchromesh took hold, manufacturers had gotten the idea about constant-mesh, helical gears and dog clutches – the kind of setup still used in most motorcycles to this day, in racing cars (with straight-cut gears), and for a surprisingly long time in certain cars like the Fiat 500 (which never received synchro as far as I know – downshifts generally needed a blip of the throttle, but upshifts were merely a bit rough rather than grinding).

    The main similarity between an old sliding-gear transmission and a modern 5-speed manual is that you operate it yourself with a pivoting stick, and that it uses gears acting directly on each other. The actual mechanical actions that take place when you change gear bear much less resemblance to each other, and the modern day iteration is perfectly fine to and relatively idiot-proof. The newest 3-shaft versions even lend themselves to dual-clutch semi-automation with the minimum of actual alterations… drill a couple holes, extend the shafts out, split the clutch into two, and shove servos on the release and selector arms…
    (OK, that’s not 100% technically correct, but you get the idea)

    In said more modern designs, the gears stay static and never go out of engagement, and so don’t need slamming back INTO engagement, which causes the grinding, chipping and balking. What moves are the selector/engagement/dog clutch rings (call them what you will), sliding along and spinning in engagement with the splined parts of the input shaft (the input-side gears themselves being freely-rotating on a smooth part of it). When you select a gear, the gearstick moves the selector arm which moves a selector fork on its own shaft, driving one of the rings towards one or other of the gears facing it. Large, rounded-off “dogs” protruding from the face of it then mate with similar outcrops (or indents) on the face of the gear and form a positive engagement for transmitting drive. Thanks to their size and shape, and the lego-brick mode of action (like jamming a stick in a bicycle wheel, rather than trying to get two bike wheels to line up and mesh nicely with each other… whilst both are spinning), this happens rather more easily, smoothly and quietly than in a crash box – though it still often requires the two sides to be turning at least roughly at the same speed. It’s easier for upshifts as the engine speed naturally falls as you take your foot off the gas (and the input shaft speed falls relative to that of the gear assembly even with the clutch depressed) and it’s just a matter of getting into the rhythm, whilst downshifts demand a deliberate raising of engine speed… particularly an issue if you’re trying to brake (doubly so on a steep hill) at the same time!

    (For some reason this isn’t too much of a problem with motorbike transmissions… even though they’re sequential, without even any neutral gaps between gears other than 1st & 2nd for you to blip the throttle in with an engaged clutch, downshifts are usually a perfectly simple press of the gear pedal… I suspect there’s some additional trickery in how they’re made, maybe with a little more play engineered in – clutchless upshifts usually come with just the slightest of jolts suggesting that; additionally, despite their higher engine speeds, there’s usually a more-than-compensating reduction gear between the crankshaft and gearbox input (with then a generally smaller step-down from output to the also-larger wheel), so everything turns rather slower, which together with the overall much lower weight (overall and per-gear) leads to rather less inertia and probably a minor auto-synchronising effect due to the whirling oil bath everything’s soaked in. All I know is I’ve tried to figure it out with mine, with the help of an exploded diagram from the workshop manual that shows how there definitely aren’t any synchros in there (vs the similar diagram for my car), and ended up just giving in and consigning it to the “life’s little mysteries” cupboard.)

    Early synchro boxes tended not to have it on 1st gear for several reasons … first being one of strength. Early synchros weren’t as strong as all that, and drivers were used to giving the lever quite a bit of welly, especially if it balked (which is to be expected when doing a difficult change with a weak synchro – just hold pressure on the lever for half a second more and it should go in…). This, along with the greater torque and rpms that would get shoved through them (even when fully engaged) thanks to the extreme gearing in 1st and the multiplying effect of a suddenly-engaged clutch meant the potential for them to break was rather higher – but plain dog-clutch engagement is very strong, which is why it’s used for race cars and for all but the most modern auto-shifting trucks (and this is why truckers have a gear-jammer reputation… they don’t have synchro and so have to become skilled to avoid dying on long downhills!). It’s also why you can identify when an old car is in 1st gear from the whining noise… the same stresses applied for the gears themselves as well as the synchros, and the one benefit helical-cut gears don’t impart, despite being smooth, quiet and efficient, is strength. Straight-cut gears are rather stronger – and noisier.

    On top of all that, in such transmissions, Reverse and 1st usually shared a selector shaft/ring, being directly opposite each other on the H-pattern, especially with 3-speeds. And you really don’t want to run the risk of the accidents or mechanical mayhem that could happen from accidentally and far-too-easily shifting directly from one to the other whilst moving at some speed… versus that, a bit of a grinding, rattling noise telling you “don’t do that, idiot!” is positively welcome. (This is why reverse is still rarely a synchro gear even in modern cars – in fact, some manufacturers even deliberately engineer it with an otherwise primitive sliding-gear arrangement, specifically so you can only engage it with any ease when stopped or moving very slowly, even if already moving backwards. This is the source of the “crunch” noise you get if you try to slam into reverse too quickly after braking fairly hard from speed… even though the clutch is dipped, the input shaft hasn’t stopped spinning yet, and the reverse idler gear is rather unhappy about being shoved into engagement with it vs the stationary output… the sound has a rapidly dropping frequency as output actually acts as a brake on the input shaft, with the idler gear as its proxy, and it all slips into place only once the input has slowed quite a bit).

    Another concern was one of cost. Synchro was both mechanically complicated, and had to be licensed thanks to the patents and copyrights. The less synchronisers you could get away with using (e.g. a single shared one on the 2nd/3rd selector ring in a 3-speed-plus-reverse box), presumably the less royalties you had to pay, as well as the fewer expensive, complicated and fiddly to produce and install parts you had to fit.

    Don’t forget that a lot of old cars were relatively slow, and also relatively low- (and wide-) geared compared to present day ones, which is why people came up with overdrives. First was usually only for starting off, low speed manoeuvring and hill climbing, and might be all over and done with by 15, MAYBE 20mph (why do you think the old automatic manufacturers thought little of habitually starting in 2nd?). General motoring was almost all achieved in 2nd and 3rd, and quite a few cars even in the 4-speed, all-synchro era were strangely proud of being able to boast that they could run as low as 10 (or even 5!) mph in top gear and still pull back to high speed without needing a downshift (even if it took a whole minute to get up to 50mph again). 2nd gear was plenty flexible enough for most low speed situations. Then – as now, actually, thanks to much better low-end torque and stronger clutches – there would be little call for downshifting into 1st on the move, rather than after having slowed to walking pace (which would allow a smooth re-engagement at idle) or a complete halt, because unless you were approaching a hill that you knew would require (a throttle-blipping double-declutched downshift into) 1st to surmount, there wasn’t that much real-world performance benefit to be had, and it would only be useful for descending the very steepest of hills, the ones that would cause you to stop briefly at the top of them and take a deep breath. Thus, there was little actual need to fit synchro, and again, it could even be regarded as a safety measure to stop gung-ho motorists from slamming the shift into 1st at far too high a speed and blowing the engine – rather more likely with 2nd-1st (or 3rd-1st!) than 3rd-2nd. If you couldn’t rev high enough in neutral to enable the speed-matched downshift into 1st, then you probably had no business being in that gear.

    Now compare all that to the behaviour of the contemporary automatics … there’s not that much of a world of difference, is there. There’s a lot of the same problems, but what the autos do is let the machinery take on all the dirty work, even if it does it about as badly as you do ;)
    These days, manuals are somewhat more developed and refined … though I’m personally not sure what to make of the rather flimsy, cable-change affair in my most recent car. It seems like a step back from the solid, positive-feeling rod change one I had to give up. Makes it harder to do a quick, smooth shift thanks to how notchy and uncertain it is. But, that’s a minor gripe really, on the whole it works fine, and demands nothing more than “realise when it’s time to shift… dip clutch… move lever… lift clutch”.

    Moving on from that, we also seem to have skimmed over the whole “epicyclic manual and semi-automatic” thing other than the brief mention of preselectors. If you consider the quite common* addition of a 2-speed rear axle to the Ford Model T, and the pedal rearrangement mods that went along with, these 4-speed variants could be sort of considered such. The “clutch” action was integral to the gear selection movement, whether by footpedal or hand lever; basically there were four different clutches (five including reverse) you could choose to engage by operating the gearshift mechanism, and going into low or reverse from a standstill involved careful take-up of engine drive so as not to stall it… but after that, it was pretty much the same as various other semiautos that came after, like the saxomat. Move the lever/pedal to the next position, and you’re in gear. With the simple addition of, say, a centrifugal clutch that would allow you to come to a halt with one of the gears still engaged, maybe a rotary-sequential selector device to co-ordinate the 2-part shifts (particularly between High-Under and Low-Over), and some device that would move the selector in response to the difference between driveshaft speed and throttle setting, it would have been a “proper” automatic with no hydraulic parts at all, and the world could have been rather different. As it was, it stayed effectively “manual” in all its guises despite strongly resembling an “AST” where the driver had to deliberately regulate the uptake of drive from rest, and command the in-range (1/2 and 3/4) shifts as well as the between-range (2/3) one.

    (* Damn it, I can’t remember the general name for them right now – as a single company made a decent majority of all those fitted – but I’ve probably seen more in-game models and example videos on youtube of T’s and TT’s with 4-speed twin-epicyclic transmissions fitted than I have ones without. Generally it was in the form of a “splitter” type gear rather than an overdrive, thanks to the T’s rather wide ratio gap between low and high; the axle would offer something in the region of a 1.66/1.00 gearing (relative to whatever the actual rear diff ratio was) to the T’s default 2.73/1.00, with a choice of either multiplication or division relative to the norm. Meaning you could have a roadster with a multiplier one giving 2.73 – 1.66 – 1.00 – 0.60, for better top speed and improved lower-mid-range acceleration and hill climbing, or a truck with a divider giving 4.53 – 2.73 – 1.66 – 1.00 for better load hauling on steep grades and less tendancy to rapidly slow to the original low-gear speeds on shallower ones… Oh, and of course, two reverse gears. Rather pointless on the roadster unless you liked scaring the crap out of your passengers, but pretty handy for the truck. Naturally, if you couldn’t be bothered with the more complex twin-shift arrangement but couldn’t afford the time/money/care to modify it into an H-gate setup, you could just choose to leave one of the transmissions in its high or low position, and only shift the other one, reverting to either a 2.73 or 1.66-to-one 2-speed modus depending which you picked (…probably safest to shift the original transmission, I would guess, as the axle would be intended to experience at least some of its changes whilst the original was in neutral going between Hi and Lo at the combined shift point?).

    Interestingly, some drag racers still use a similar sort of system, when uprated Powerglides and such no longer suffice – a bunch of daisy chained 2-speed epicyclics, with a single clutch that locks them into “forward reduction” when engaged and “direct” when released, and a regular plate clutch (or heavy duty “dumb” TC) at the head of it all. Bunch of little levers in a row forming the control panel. Shove them all (say, 2, 3, or 4 of them depending on your power and expected top speed) forwards, roll to the start line, do your burnout… rev up… lights go green, dump the clutch, then over the next three seconds or so rapidly slam each lever down in turn, jumping up through the ratios quickly, seamlessly – and completely manually – as each reduction clutch is released. Usually a uniform sequence is used, but there’s no reason a roadable dragster couldn’t have a back-and-forth type splitter gear in there which could be rocked one way and the other whilst the other, wider gears were being mostly changed in just one direction.

    And, well…

    All this sort of begs the question…

    Given how the gear selectors in my bike box are quite happily synchronised in their operation by a single up/down footpedal despite the box internally more resembling some notional 5-speed version of the Fiat 500 (three different selectors, whose forks move back and forth just as if they were being pushed and pulled by a regular lever in an H-gate)… and operators of the Ts and TTs with 2-speed axles presumably didn’t suffer badly from jolty shifts even when having to personally synchronise a pedal and a hand lever… and even just now I manage to envisage controlling and tightly synchronising the latter using the former’s cam-like peg-in-groove shift barrel and some simple adaptions to make the output act on brake bands instead of dog-clutch selector forks…

    …how come the makers of the AST and the 4-speed Hydramatic couldn’t dream it up? Heck, the former was almost there, they just needed something to hook up that 2nd-3rd change.

    No complex hydros, no strange lag, anything like that. The barrel turns, the pegs located in the matching grooves are driven one way or the other, and the attached mechanical parts are forced to move in perfect synch. In fact, the shifter itself will balk and refuse to move unless they do, as unsynched movements will jam it before it can turn any further (an actual safety feature on my bike uses a torque-sensing ratchet pawl to do that on purpose to prevent you downshifting more than two ratios in one go without letting the clutch back out, as a 3-gear jump would generally represent a difference in rpms that would cause engine damage, dangerous loss of rear wheel traction, or both; soon as you let the engine hook up for a split second, it frees right back up). Very easy to control with an automated system because all it needs to know is whether there’s a need to go “up” or “down” from it’s current condition (say each shift takes half a second, and it needs to jump to 1st from 4th; “need to go down”… it shifts 4>3 … “need to go down” (still)… shifts 3>2 … and there’s still that pesky “need to go down” signal, well ok let’s go 2>1… and hey, that was “only” 1.5 seconds, probably quicker than a human would have figured it out!), and what the limits are (hi/lo ranges, or e.g. “S” or “2” position – and of course, the point at which 1st becomes Neutral becomes Reverse). The only outputs from whatever control logic is used – mechanical, electrical, hydraulic – would be “up” and “down” from the current position (and, if it’s an actively clutched setup rather than a fluid/centrifugal/TC one, some kind of release bearing servo). Could even add a tiptronic-style sequential manual shift option (either with positions 1-4, or a ratcheting push-up/push-down motorbike/flappy paddle setup), and have Park as something triggered by pulling the lever sideways from N.

    But then, I am looking at this with 20/20 hindsight and the eyes of someone who’s already versed in everything that has come along since. I have a feeling bike manufacturers themselves didn’t hit onto the sequential/barrel shifter idea until after WW2 anyway. And although it could have been achieved just as easily with cams on a rotating shaft (produced using the same tooling as the engine camshafts!) actuating other parts in synch, and it’s almost certain the Victorians did similar things in order to make steam engines work properly, both of those would probably have been viewed as primitive, old tech solutions that should be left in the coal-burning age rather than as practical options for the modern early-20th-century gasoline-burning automobile maker. Hydraulics are where it’s at, don’t you know! So much better than cams and cables and all your old rot. It’s alright, we’ll work this out sooner or later…

    1. I ride motorcycles and generally do the upshifts without the clutch. I can also do downshifts without the clutch but feel that that is more likely to cause problems which I prefer to avoid.

      Motorcycles with chain drive have a cushioned sprocket on the rear wheel. I’m sure that that helps to smooth out clutches shifts. Also, the dog clutches have considerable play which is obvious if you try rolling the bike forward and backward while in gear with the clutch engaged. That play no doubt helps the dog clutches to engage even when there is a significant speed mis-match. The lighter rotational inertia of motorcycle engines also helps, at least on sport bikes or similar bikes. On a bike with a big heavy engine, such as a Hardly Davison, clutches shifting might not work so well.

      On cars, there seems to be very little play within the manual transmission so speeds must be pretty well matched before engagement can occur. The synchronizers will block engagement attempts until they have forced speeds to match.

  33. My father’s first car with automatic was a ’53 Pontiac–with Hydramatic, of course. He was a buttoned-up guy and usually not prone to getting openly excited about things. For weeks after getting the Poncho, he was telling people, “You just step on the gas, and it goes! You step on the brake, and it stops!”

  34. Automatic transmissins in Indian traffic conditions have not proved effective due to high congestion oftraffic, unruly traffic and slow moving traffic too. Frequent shifting of gear ration result in fast wearing out of brake bands and aslo clutch plates of interlocking clutches. Slow speeds cause slippage in fluid couplings leading to poor fuel economy. Thus Indians mostly prefer manual shift gear boxes. Earlier we had ceilo a korean car with auto transmission that failed in indian conditions. Many customers got manual shift gear boxes fitted.Ofcourse some woman driversprefer auto gears and but are limited.

    1. Automatic transmissions have generally been slower to catch on in markets where fuel prices, tax considerations, and available income favor cars with smaller engines, in part because automatics have traditionally been a serious handicap for smaller engines in performance and fuel economy.

      I’m not terribly familiar with the Daewoo Cielo, but if I understand it correctly, it was essentially a restyled version of a car sold in the U.S. as the Pontiac LeMans, fitted with a smaller 1,498 cc engine and offering a rather elderly three-speed automatic. With only 94 lb-ft of torque, I imagine the automatic didn’t do much for either acceleration or fuel consumption.

  35. I had a 64 Olds 88 in the early 80s. The HM was worn and 1st gear overran. I was annoyed to learn the HM had no exxternal means to adjust the bands. One trans mechanic said they basically self -destruct.

    1. The ’64 Eighty-Eight did not use the original Hydra-Matic, but the later three-speed torque converter version, introduced in 1961. (That transmission is described in Part 2.) One of the things the later Hydra-Matics attempted to address about the original four-speed H-M was that it was very sensitive to proper adjustment of the bands — if they weren’t set up just right, it would throw off the shift timing and make the shift jerky, which was the cause of many complaints.

  36. Very interesting article and comments.

    Does anyone know where I might find a new Speedometer Pinion for a 1948 Cadillac Fleetwood 60 Special Hydra-Matic Drive?

    Thanks for any information anyone can provide.


    1. I don’t sell parts, so I’m afraid I can’t help with that. Sorry!

      1. Looking for a Transmission for a 1954 Lincoln cosmopolitan 317.6 engine 2door H/T. Live in Prosperity, West Virginia. Two(2)persons have tried to repair this transmission and have failed. This car is 964 off the line when built at the Michigan plant and all is original. Anyone one out there that can help???? Would be greatly appreciated. Thank you.

        1. I’m afraid I can’t help with parts or repairs, sorry! (Also, I redacted your phone number and ZIP Code; if you’d really like to post them, I’ll put them back, but I wouldn’t recommend it. I don’t know about you, but I get enough junk phone calls!)

  37. Would anyone know of a modern transmission I could use to replace the original transmission in my 1950 Series 62 4 door sedan.
    Any help greatly appreciated.

    1. I’m not able to help with modifications or repairs — sorry!

  38. W. G. Wilson, did I miss mention of him?

    Even worse a complete omission of that genius of the 20th century Frederick Lanchester!

    1. Wilson is mentioned, albeit briefly, Lanchester is not. This article is about the origins of GM’s Hydra-Matic; it is not and does not purport to be a complete history of the automatic transmission or the fluid coupling, which would be an account several times the present length. The Wilson and Cotal preselector transmissions represent a separate (if not unrelated) and complicated subject in and of themselves.

  39. I have a 1954 Rolls Royce Silver Dawn that uses the “Hydra-Matic” built by RR under a license from GM. The transmission was replaced under warranty in 1964, at about 75,000 miles, and the second transmission is still doing fine at 150,000 miles. An interesting feature is a servo-power takeoff on the side of the transmission that provides mechanical power assist to the braking mechanism. A very complicated set of levers and rods tying that all together but I’ll have to say it works very well.

  40. Wow, what a great discussion of this great Automatic transmission. I had 3 cars with the Hydra-Matic, a 1950 Oldsmobile, a 1955 Chevrolet with a 394 Cu. In. Olds engine & another 1955 Chevrolet with a 427 Chevrolet Engine. I was totally fasinated with the Hydra-Matic! I used to study the Hydra-Matic’s power paths shown in the motor’s manual. That really fascinated me! After studying those power paths a bit, it became obvious to me, that the 2-3 shift was VERY complicated & required exceptional timing & coordination control to make that shift feel smooth! Jim Geiger’s post above explains this very well. Some of the timing trickery used by the Hydra-Matic was the 2½ turn front band & a drilled hole in the side of the front band’s apply piston bore to give a timing signal to the valve body to aid the timing of the other 3 frictional elements that were ether applying or releasing at that moment! A mis-adjustment of that band will cause 2-3 shift problems. Has anyone seen the Hydra-Matic’s used in some WW2 tanks? They had a deep, cast iron oil pan with cooling water ports in it.

    1. I’ve only seen the AFV version in photos, but I’m still amazed they made it work. Not at all an obvious idea, although I can see obvious benefit to not having to manually change gears while driving a tank with people shooting at you.

      Hydra-Matic is one of those devices that belies the idea that mechanical analog systems are simpler than electronic ones. There’s a tendency to romanticize the purely mechanical as always meaning simple, rustic, rugged devices that even the local blacksmith could fix for you, but with stuff like the early Hydra-Matic (or most mechanical fuel injection systems for street cars), that’s obviously not the case!

      It would have made things a little easier, I suspect, if the original Hydra-Matic had used one-way clutches rather than bands, but Oscar Banker’s patents in that area would likely have made that difficult, which may be why GM didn’t go that direction until the fifties, after the applicable patent terms had run out.

  41. GM still builds Automatic transmissions with potential timing problems. Their popular 700R4 (82-93, Hydraulic control) & the newer 4L60 (basically the same transmission with computer control), if the 2-4 band releases before the 3-4 clutch applies, it feels like neutral & if the reverse happens, it goes into 4th. (overdrive) momentarily.
    Must be some powerful reasons to build a transmission that way. Economics? Compactness? Efficiency?

    1. Bands do have their advantages. The limitation of using a one-way clutch as a brake is that unless you do something clever and complicated, its behavior is completely dependent on load conditions. Brake bands allow positive control over when the brake engages or disengages, which allows more flexibility of transmission programming (whether analog/mechanical or electronic). Of course, the trade-off is that you then HAVE to control the engagement and disengagement and manage the timing and things can get wonky if the timing is off.

      1. Also — and this is a point I’ve only recently come to grasp, working on revamping the article about later GM automatics — is that one-way clutches will automatically disengage if the driveshaft overruns the engine, which is a big problem in mountain driving, particularly with vintage drum brakes. Later Hydra-Matics (both Controlled Coupling Hydra-Matic and the three-speed Roto Hydra-Matic) dealt with this by also providing bands and/or disc brakes that worked in the lower gear ranges (for example, in a dual-coupling Hydra-Matic, selecting D3 or S range will cause the rear band to engage in second gear), but that’s an expensive way to go.

  42. I had a 55 Holiday Super 88 with a strong 4bbl rochester. The old car had 80k but ran stronger than anybody else. The trans shifted and was done before friends had their foot on and off the clutch. Solid off the line feel was impressive. Most of us teenage drivers loved the strong. shift feel. That detent downshift was brutal and should be illegal but really raised friends admiration. Yes, that trans is for the teens as far as I am concerned. I even snapped a driveshaft companion flange with the kickdown.
    If the start battery was low the ole car could roll down the grocery store parking lot slight incline then start at 6 mph when taken from neutral. What an impressive machine for that ere thanks to Highly-Dramatic. Nobody said things like “Slip and slide with Powerglide” or Dyna-Slow. Heavy as it was I felt like I had a real working machine under me as I drove that car. Never had another like it.
    NOSTALGIA from 1964. Rich kids had a new GTO, but I had the Olds HYDRAMATIC 4 speed with 2 clutches and 2 planatary gear boxes to make 4 cleverly arranged gear results. I knew the guts of that car! 324.2 Cubic Inches with Rochester 4GC and Four speed Hydramatic solid shifter that is good enough for trucks.
    So much for my feelings about automatics.
    Thanks for listening!

  43. Aaron,

    Great article! But just a question about synchromesh gearboxes. You cover the design of the original synchromesh gearboxes, but there was a later (and very clever)improvement called the Baulk Ring Synchromesh, which is what is used in all manual transmissions (except motorcycle trannys) to this day. As I understand it, this was originally designed by Porsche in the 1950s. Is this correct?

    1. Yes — this point is briefly mentioned in the article. I didn’t look up the patents for the balk-ring system — I didn’t want to get yet more sidetracked — so I don’t know the numbers or in whose name they may have been filed (although I assume the assignee was Porsche AG). However, it was written up in some detail in the automotive press around 1951 and you could probably look up the relevant patents relatively easily if you’re curious about it.

      1. Thanks Aaron….. I must have missed the mention of it in the original article.

        1. That point was added with the recent revision, so it was a new addition. If it makes you feel any better, with articles this long and this much revised, I sometimes end up having to search on a particular article to see if I actually included some detail or not!

  44. Congratulations for this great article.
    I live in Brazil (where transmissions are mostly manuals). Anyway, here, “hidramatico” is synonymous of automatic transmition, obviously due to the GM hidra-matic!
    99% of the people here don’t have a clue that it was a kind of auto box!

  45. Aaron, I enjoyed reading through your comprehensive discussion of the development of the Hydra-Matic. My introduction to these boxes came in 1950, when, as a teenager, I bought a rather clean 1941 Olds 98 with an inoperative H-M for about 1/4 of what it was worth working properly.
    I had a 1949 Motor’s manual, which had a step-by-step guide for a 1948 Pontiac to work with. The failure point was quite obvious—the bronze cross-drive gear that drove the front pump was stripped. There were a lot of people who thought this project was doomed to failure, and my success led to a job with Packard in 1954, dealing with the gear-start/twin Ultramatic problems. That’s all a long story, but the important part was that I learned some things about the H-M from Forrest McFarland and a few others, and later was involved in transmission shops where our bread and butter was H-M’s and Dynaflows. I’ll toss out a few comments on the 1937-39 semi-automatic and 1940-56 Hydra-Matics.

    On the semi-automatic, “neutral” was in the head-end gearset, along with “forward” and “reverse.” With the car stationary, the single oil pump did not move. Both servos were spring-applied, so the gearbox was in 1st gear, and could not upshift until the car was in motion.
    The most comprehensive repair manual for these units I know of was in the 1939 Olds shop manual.
    Starting off in Hi range, which gave a shift pattern of 1-3-4, was hard on the box, and at low speed, oil pressure would drop, causing the unit to hunt between 1 and 3. The spec oil was “the same as is used in the engine,” which made shift quality quite sensitive to oil temperature and viscosity. Also, with no torus to cushion shifts, they were pretty abrupt, although with only 3 pistons instead of the 6 used in pre-1944 H-M’s, clutch application was slower and had less clamping force. As to the number produced, a 1942 Motor’s Flat Rate manual, which also lists parts, indicates that the third (and last) pump was introduced in 1938 at s/n 26482.

    Going on to the 1940 H-M, one reference I have that you did not list is “1940 Oldsmobile Hydra-Matic Drive Service Instructions,” Oldsmobile Division, General Motors Sales Corporation (no date). Provenance is “Library, General Motors Research Laboratory.” This is not the shop manual, but an introductory 144-page manual that also introduces some other changes between 1939 and 1940 cars. It cites “30,000 (1937-39) automatic transmissions…” A rather salient point in the text is that while the H-M is different from the semiautomatic, it is being introduced as a second generation unit. It is also specific that dealer shops will need to have at least one person prepared to service the units, and presumes that each shop will have someone already familiar with the semiautomatic. Explicitly stated is that Olds will not provide exchange new/rebuilt units, as they had done with the semiautomatic.

    Now, to a few points that need some amplification:
    The prewar units (both the semiautomatic and the H-M) used alternating bronze and steel clutch plates. The steels were coned, to provide some cushioning on make-up; but, more importantly, to provide release pressure forcing the “sandwich” to open up. A major service issue with this was that the steels would flatten out after a while, causing the clutch to drag when clutch pressure was released. This generally affected the front clutch, so the upshift pattern would be 1-2-4/3-4, which jerked the bands and U-joints and broke parts. Raising the throttle pressure a bit generally gave a better 2-3 release of the front clutch.

    Prewar units did not have an annular clutch piston. Instead, there were six round pistons that slid into pockets acting on a cast pressure plate. There was no “check ball” provision for draining clutch actuation oil—it all had to go back through the oil delivery sleeve. Thus the need for a real “kick” to force the plates apart.
    I’ll note here that the (Raybestos-developed) lined plates that were introduced (with the annular clutch pistons) in the 1944 revision were bonded to “wavy” spring-steel plates that gave a much more positive and permanent release force, and were specified for service in parts books.

    The governor valves were hydraulically balanced against line pressure (fixed 80 psi), rather than spring balanced. Thus, if one of the governor valves stuck, hydraulic pressure would force it to move.

    Before 1952, downshift was 3-1, skipping second speed. A forced downshift at 10-15 mph, when the throttle was opened, was a real lurch. Keep in mind that the rear servo had an accumulator with a check valve that delayed application of a released rear band by a couple of seconds, so that a coasting downshift did not generally give a sudden drag. However, this made the 3-1 downshift more of a lurch, and required a pause in a forward range when shifting from neutral to reverse.

    Prewar units used a cross-driven front gear pump in the front servo, with limited capacity. That, coupled with a spool valve regulator, both near the oil pan bottom, produced a pronounced whine until around 20 mph, when the rear pump pressure came up. Oldsmobiles were particularly loud. The Cadillac units had a 3-gear set, with an idler on the drive gear, which gave a different lower-pitched whine. With any wear, the transmission would “lock up on reverse coast” when the spring-applied rear servo applied the band, and allow the front band to release at idle as well.

    The shims under the reverse pawl bracket are not there for mechanical clearance, but for timing.
    To get to reverse from neutral, the rear band had to apply to stop the drum from spinning, but to get the reverse pawl to engage, the band had to start to release just as the pawl was entering the reverse unit teeth.

    The loose steel plug at the back of the governor sleeve is there to balance the force of line pressure at the front of the sleeve. It pushed on the side pan to reduce the fore-aft force on the sleeve.

    Band release was by application of clutch pressure to the servo. The front servo was “always on” except in neutral, and released by applying clutch pressure to a larger area than on the apply side. This prevented runaway (“flare”) as the front band wore—the band would not release until the clutch was actually engaging. the rear band/clutch operation was similar, except that the band was spring-applied.

    I’ve noted a spring 1944 date for introduction of major changes in the H-M. As has been discussed, H-M production continued through WWII for tanks and personnel carriers, and there were plenty of developments that were incorporated into what was a “second generation” H-M. Notable are:
    1. Front pump—a much larger crescent pump and simplified pressure regulator now mounted in the front cover, concentric with the shafts.
    2. Case—one common case for all H-M’s. Reverse pawl now mounted on a separate bracket that would break free if overstressed, rather than cracking the case.
    3. Reverse blocker—incorporation of a piston pressured by the rear servo accumulator that prevented movement of the shifter beyond Lo until the rear band was fully applied.
    4. Clutch redesign. Annular clutch pistons with soft lip seals replacing the six-piston/pressure plate setup. Plates were now flat steels and lined spring steel wavy plates.

    Going into postwar H-M’s, there were two standard drive trains, “big” and “small” which fit in the same case. The “small” units had three planets in the planetaries; the “big” had four. Also, the “big” units had more clutch plates. “Big” units went into larger-engine cars (Cadillac, Olds V-8, Lincoln, Hudson Hornet). GM published configuration guides outlining these differences.
    Indeed, GM published a lot of material targeted to independent shops to encourage them to do automatics for profit—in the 1950’s they were not at all secretive about servicing these units outside the dealer network. Much of the actual “tuning” of the units came in the torus member configurations. 1949 Olds V-8’s had full-vaned torus members, and were notoriously heavy shifters. For 1950, they reduced the torus member vane areas, making for much softer shifting. 1950-51 Olds H-M’s were built with a second gear start in Dr, obtained by changing the springs in the 1-2 shifter valve. That caused a problem because with the specified 375 RPM idle, the torus now spun at 375 RPM instead of 258. They went so far as to offer a service bulletin and kits to return the units to the normal 1st gear start. 1950 also saw the introduction of modulated main line pressure.
    In 1951, saw the introduction of the hydraulic cone clutch reverse brake. The old pawl setup was retained, but it now had a windup spring and positive blocking so that the pawl would only engage as a parking lock. 1952 changed the valving—not only providing a 3rd gear “Drive” range, but adding valves to allow a 3-2 downshift.

    One other change between prewar and postwar was to move the detent spring-loaded “clicks” from the shifter control head in the car to the valve body in the transmission. That was important if you were going to retrofit a postwar box into a prewar car. In general, that was a fairly straightforward swap particularly with Oldsmobiles.

    As to oil to use in an H-M, all of the old boxes that ran on Type A—use Dexron.

    As a side comment, when it comes to automatics and efficiency, the Ultramatics (and the Studebaker first type B-W automatics) were not inefficient gas hog slush boxes. Both had converter lock-up clutches and were direct mechanical drive, just like a standard transmission, for most driving.

    1. Hank,

      Thanks for the info. Some of this I had found (like the alternating bronze and steel clutch discs) and omitted both in the interests of simplification and in the vain hope of discouraging folks from thinking I know how to fix or rebuild these transmissions, which I definitely don’t! The 3–1 downshift I had not been clear about, since a lot of Hydra-Matic references talk mostly about Dual-Range and don’t address the differences in the earliest units.

      Regarding the introductory instruction manual for 1940 units, I would have loved to see that in revising this article! I spent a LOT of time looking for specifically for a 1940 or 1941 manual, without any luck. (The closest I might have found was that the library had a 1944 Cadillac training manual, which I decided not to go through the hassles involved in accessing, since it wasn’t going to answer most of the questions I had about the early Oldsmobile units.)

      I’ve done some work on a followup item about split-torque and lockup clutches, referencing the Ultramatic and Borg-Warner DG, although I keep getting sidetracked.

  46. I’m working on two Hydramatics from a 1945 M24 Chaffee tank, which is powered by two flat-head V8 Cadillac engines. The Chaffee was considered a light tank, weighing in at around 20 tons. It was fast and agile for its day.

    Doing research on the HM, I came across this excellent information. My contribution is to list the unusual features I have found on the model 256T HM’s that were built specifically for the Chaffee:
    – they have no reverse gear. Reverse is in the transfer case that connects the two drivetrains to a common driveshaft. This makes the transmission very simple and very short.
    – they don’t use the rather finicky linkage from the carb to the valve body to provide an engine load signal. Instead intake manifold vacuum is supplied to a “throttle vacuum control piston” that moves the throttle valve in the valve body, similar in concept to the vacuum modulator used on much later transmission types. I wonder why this design did not catch on post-war? It seems much simpler.
    – the manual control rod from the shifter comes in at the back of the side cover and moves the manual valve linearly, unlike the rotary, detented linkage entering the middle of the side cover. This may have been because the drivetrain is back-to-front; the engine is at the back of the tank, with the HM in front of it.
    – they have a massive flat-tubed cooler in a very deep, cast iron pan. The cooler is located in its own sealed-off compartment. Coolant flows over the outside of the cooler and oil flows through the tubes. This requires very reliable sealing between the coolant- and oil-filled compartments in the pan.
    – all the internal drive train components are standard parts, such as were fitted in cars of this period, but I did read an article that said GM expended much effort improving these transmissions in subtle ways, for example machining a generous radius on parts susceptible to fatigue, using high quality steels, and hardening areas subject to wear. I assume post-war cars benefited from this.

    Malcolm Towrie

    1. Malcolm,

      That’s fascinating — I’ve looked at the technical manuals for the M5, but not the M24, so I hadn’t realized it differed so much. On the latter point, that’s probably a safe assumption. Cadillac engineers have said that with production-spec equipment repurposed for military use, they had to make a variety of refinements to satisfy Army inspectors, and the postwar Hydra-Matics were more reliable than the early units.

      GM did start taking an interest in vacuum modulation for transmission control after the war, although not on Hydra-Matic. The first 1950–1952 Powerglide transmissions (the dual-impeller variety) used manifold vacuum to adjust line pressure based on load, a feature that gradually spread to other transmissions. I wonder if Detroit Transmission was wary of it for Hydra-Matic for reliability reasons. The mechanical linkage was fussy, but I imagine its failure tolerance was higher (it would work, albeit badly, if out of adjustment, and could be fixed more easily in the field than a vacuum leak).

  47. I’ve recently, through YouTube, become aware of an interesting variation found in some Hydramatic equipped, 1953 Kaisers, but not all of them. Some of them evidently had not only a Dual range Dr position on the shift quadrant, but also a dual range L. This is not just a mistake on my part. The car demonstrator in the YouTube video also mentioned it in his description of the car. This prompted me to verify it visually. I surmise that one of the tick marked L positions allowed a 1-2 shift, while the other started the car off in second gear, as was normal for the low range in Hydramatic equipped cars I’ve driven from ’52 through ’55 model years.

    Another interesting anomaly: Some factory fresh 1956 Pontiacs and Olds I drove back in the day, seemed to be equipped with truck Hydramatics. These particular cars did not have the new dual coupling transmission, but had harsh, unpleasant shifting through all the gears, exactly like truck Hydramatics but not typical in cars through 1955.

    1. Interesting — I’ve never seen a dual-low-range Hydra-Matic, although it would be a logical development. You also reminded me that I didn’t mention the second-gear start in Low, which was a new feature of the Dual-Range Hydra-Matic. (I added that point to the text.) With the regular Dual-Range transmissions, full throttle would still give you a first-gear start, which seems like it would occasionally be counterproductive, given that the whole point of the second-gear start was as sort of a poor-man’s traction control. A mechanism to let you decide definitely whether you wanted to start in first or second would seem useful.

      Regarding the 1956s, the older single-coupling Hydra-Matic continued to be available in Oldsmobiles and Pontiac into the 1956 model year. I don’t think it was a truck Hydra-Matic, just the existing Dual-Range unit. As for it feeling harsher, I can see several possible explanations. One of the rationales for the dual-coupling transmission was that it had greater torque capacity, so the final single-coupling transmissions may have been firmed up to deal with the more powerful ’56 engines. Another possibility is that the transmission mounts of the ’56 cars were tuned for the smoother new transmission, leaving the older transmission feeling jerkier than ever. (My assumption is that the only reasons for the overlap in single-coupling and dual-coupling transmissions were to clear out stocks of the older unit and perhaps to cover any shortfalls while production of the new transmission ramped up.)

      1. “(My assumption is that the only reasons for the overlap in single-coupling and dual-coupling transmissions were to clear out stocks of the older unit and perhaps to cover any shortfalls while production of the new transmission ramped up.)”
        This might be true. Every 1956 Oldsmobile that I have heard about or 1956 Olds HM transmission that I owned, the transmissions were name-plated R55, for the 1955 production year, I never heard or saw one labeled R56, for the 1956 production year. These R55’s were used in just the 1956 Olds 88’s, not the 98’s. But there is a possibly that you could special order the DR HM in the 98’s as they were still available. I personally have never seen a 1956 98 with a DR HM trans.

        1. I agree. The ’56 brochures I’ve seen indicate that the new Jetaway Hydra-Matic was standard on the 98, which would suggest you’d really have to twist someone’s arm to get a 1956 ’98 with the D-R unit. I suppose it’s possible — if I ran into one like that, I wouldn’t necessarily disbelieve it was authentic — but I can’t see any reason why someone would go to that trouble, so it may well be that there just weren’t any 98s built like that.

  48. I have been reading about the original Hydramatics here, and I would like to know how prior to the “dual range” hydramatics introduced in 1952 could a hydramatic car be downshifted from fourth to third gear in order to get engine braking going down a long, steep, hill? My Dad owned a 1954 Buick Special with Dynaflow transmission, which was super smooth and at speeds under 55 mph you could shift the gear lever from Drive to Low to downshift and get engine braking going down a long, steep hill to save the brakes. Didn’t the original Hydramatic cars have a problem not being able to manually downshift into third gear going down a long, steep hill to get engine braking? Low position was only first and second gears and you couldn’t go faster than 40 mph or so. So what if you were going about 55 mph down a hill, you had no way to get engine braking with the original hydramatics?

    1. I don’t think so — you could kick down from fourth to third, but I don’t think there was any way to manually hold third. So, the Dual-Range Hydra-Matic was a great advance in that respect. (I’m somewhat surprised they didn’t do that earlier; the idea of a third-gear hold is something Earl Thompson had originally proposed for the Automatic Safety Transmission back in the thirties!) There was some engine braking in direct drive because of the split torque arrangement, although being able to hold third would obviously have been preferable.

  49. Thanks very much for your reply, Aaron. I always thought the Cadillac engineers made a huge mistake not finding a way to shift into third gear and hold it for going down a steep hill somewhere in the Colorado mountains. I noticed Rolls Royce when they acquired Hydramatic immediately made all four gears available to the driver with the shift lever. Speaking of Dynaflow, that 1954 Buick was in our family for 13 years and we never had any transmission trouble. I think that the torque converter in Dynaflow was a superior design that later ended up in all automatic transmissions by the end of the 1960’s.

    1. The Rolls-Royce version of Hydra-Matic was the 1952 Dual-Range unit. Rolls-Royce manufactured it themselves (except at the very beginning), but apart from generally higher manufacturing tolerances, it was functionally the same as the ones used by GM and GM customers from 1952 on. The Dual-Range Hydra-Matic didn’t QUITE give you complete control over all four forward gears, although it was close enough for most purposes.

      Hydra-Matic’s later development was mostly the responsibility of Detroit Transmission Division (taking some concepts from the Engineering Staff transmission group); Cadillac was essentially just one of their customers. My guess is that Detroit Transmission probably started thinking about revising the control layout around the time the Livonia plant was being built or perhaps a little afterward. Although Thompson’s group had proposed a third-gear hold years earlier, the original design team had been under some pressure to get the initial version ready for production, so I imagine a number of “nice to have” features were shelved at that stage for expediency. By 1949–1950, Detroit Transmission had lots of service experience and feedback from various Hydra-Matic users, including GMC, to guide future development.

      The torque converters used in the Dynaflow series — there were four major versions, as discussed in the sequel to this article — have very little resemblance to the ones in later automatics like Turbo Hydra-Matic. (When the latter was introduced in 1964, some Buick press and marketing materials implied that it was basically a Hydra-Matic transmission with a Dynaflow/Turbine Drive torque converter, which is really not true at all and falls apart immediately if you actually look at the comparative schematics.) As it turned out, none of the major early GM transmission families ended up representing the direction of automatics of the sixties and later. The prescient models were the Borg-Warner/Fordomatic transmissions (three-speed Ravigneaux gearset with torque converter) and TorqueFlite/Ford Cruise-o-Matic (three-speed Simpson gearset with torque converter), all of which used a straightforward three-element torque converter. (The original Dynaflow had a five-element converter with a two-stage impeller and dual stators; the early Twin Turbine unit in your family’s car had a four-element converter with a single fixed-pitch stator with twin turbines driving an integral planetary gearset; and late units had a five-element version of that with both fixed-pitch and variable-pitch stators.)

      By the way, if your comment doesn’t post right away, you don’t usually need to repost. I have comment moderation turned on to avoid besieging you all with the mountains of stupid spam I periodically get, so I have to manually approve comments before they appear.

  50. Thanks again for all your info Aaron, I’ve learned a lot from reading your comments. You are right that Rolls Royce did indeed use the Dual-Range Hydramatic. In a picture of the gear shift on a Silver Cloud steering wheel, I can see “N 4 3 2 and R”. Obviously, 4 gave you all four gears, 3 gave you just third gear for engine braking on a hill, and 2 gave you the first two gears, just like on Cadillacs and Oldsmobiles of 1952. I’m also glad you explained that modern torque converter transmissions are based on the Borg Warner design, which was the basis for the Fordomatic and Mercomatic three speed automatic transmissions of the early fifties. I had always erroneously presumed that Buick’s Dynaflow was the basis for modern Turbo-Hydramatic transmissions. But I still think Buick deserves credit for having one of the first successful torque converter transmissions, with Chevrolet’s Powerglide coming shortly afterwards. Powerglide transmissions, incidentally, are still widely used by drag racers because of their tremendous durability.

    1. The Dual-Range unit actually still gives you all four gears in D3 (or 3, or however it’s labeled on the quadrant) — it just raises the shift point much higher. The way Hydra-Matic (and most pre-electronic automatics) shift gears is with a series of shift valves, which are held closed by spring loading and throttle valve pressure (hydraulic pressure metered proportionately to the position of the throttle linkage) and forced to the open position by pressure from a centrifugal governor on the tail shaft; the wider the throttle is open, the more pressure the governor pressure it takes to execute the shift. In D3, pressure is applied to the “keep closed” side of the 3–4 shift valve equivalent to the throttle being wide open. The transmission will still shift to 4th (keeping the engine from overrevving), but it will do it at a much higher speed (generally in the region of 65 mph, depending on axle ratio). Low or “2,” surprisingly, would actually sometimes let you hold 2nd. If you started in Low, the Dual-Range transmission would start in 2nd gear and stay there, although if you were going slowly enough, you could kick down to 1st.

      I don’t mean to imply that all modern transmissions are based on the Borg-Warner model (of which there were a number of significant variations) so much as that and TorqueFlite were much closer to the typical automatic of the seventies or early eighties than either Hydra-Matic or Dynaflow were. Hydra-Matic and Dynaflow are very odd ducks that got a series of belt-and-braces upgrades over the years to make up for their inherent limitations. (You might be surprised to know that the people who developed the concepts behind Dynaflow did not consider the fact that you could and frequently needed to manually select Low either advantageous or desirable! They didn’t want to need any driver control other than forward, reverse, neutral, and park or to have any perceptible shift points at all.)

      1. According to what I’ve read, if the Dynaflow is often started in L, problems will result because the Dynaflow was never designed to be started in L. L was supposed to be used as sort of an emergency gear rather than regularly. The same was true with the early Ultramatics. Exactly what problems would result from frequently starting in L I don’t know.

        At one time I owned two antique Packards with Ultramatic: a 1953 Cavalier, and a 1955 Caribbean convertible. The 1955 Ultramatic was much better because it provided the option of automatic low gear starts.

        1. It’s not that starting in Low was intrinsically bad, but that the designers initially assumed that you would normally operate in Drive, with the high clutch always engaged, and stressed the clutch pack and low band accordingly. Transmission line pressure in Drive was also about half that in Low or Reverse, so pressure would drop off as you moved to Drive.

  51. Thanks again, Aaron. You really know your stuff. I’ve learned more from reading your comments about Hydramatic and Dynaflow
    than in several years of reading about them from other sources. It’s a subject that I’m very interested in because my Grandfather had a 1949 Pontiac Chieftain Six with a Hydramatic transmission that I actually drove (it’s still the oldest car I’ve ever driven).
    I can remember him and my father arguing about what was the better transmission, Hydramatic or Dynaflow. My grandfather felt that since Cadillac had Hydramatic, it must have been superior to Dynaflow. But from driving the two vehicles, I noticed that under full throttle from a complete stop, the Pontiac had a very unsmooth first to second shift, whereas my Dad’s Buick was always very smooth under hard acceleration, plus you could get engine braking by downshifting to Low.

    1. That was the tradeoff: Dynaflow was extremely smooth because in Drive, it was functionally a continuously variable transmission. (In the twin-turbine units, there was a planetary gearset within the converter, but it didn’t create any perceptible shift points.) However, maximum gear multiplication in Drive was, on a 1954 Twin Turbine unit like your father’s, 2.45:1, which was (numerically) less than 2nd in Hydra-Matic. In a 1949 Hydra-Matic, 1st gear is 3.82:1, which gives you much better off-the-line acceleration, but also means a big ratio drop from 1st to 2nd, which emphasizes the mechanical shift shock. (The 1–2 shift on a single-coupling Hydra-Matic engages the front clutch, which locks the intermediate shaft to the front sun gear drum.) Hydra-Matic was probably also a little thumpier because the transmission (or more specifically the front ring gear) was actually bolted to the engine flywheel. In Dynaflow or most other fluid clutch transmissions, the fluid coupling or torque converter isolates the planetary gearsets from the engine, which provides an additional cushioning effect. The Hydra-Matic way is much more mechanically efficient, but you don’t have quite that same isolation.

      1. The trick to getting decent pickup in the 1954 Buick Special was to start out in Low and then upshift to Drive at 55, which I did quite often on my way to Fairfield University. You got about 14 or 15 seconds zero to sixty, which wasn’t too bad for a 3800 lb. car. My grandfather’s 49 Pontiac Chieftain could cruise easily at 60 but top speed was only about 75 with the 93 HP six. But that car was built like a tank and it outlived my grandfather and my uncle kept it until well into the 1970’s with no rust.

        1. If you started in Low, the early Twin Turbine Dynaflow would give you a maximum starting ratio of 4.46:1, which would then fade to 1.82:1 at higher speeds. So, you can see why that would give a lot more snap. Of course, then you had to shift by hand, and Dynaflow’s clutch pack and low band weren’t really designed for routine use.

    2. What was the second-to-third shift like? That was the one that had the reputation of being abrupt.

      1. The full throttle second to third shift in the 49 Hydramatic Pontiac Chieftain wasn’t too bad, it happened at about 30 mph, and then it got up to 50 mph in third gear but wouldn’t go any higher than that. I finally lifted the accelerator and it shifted up to fourth and I got up to sixty with no trouble. What I noticed most about driving the 49 Pontiac compared to my Dad’s 54 Buick was the very high driving position. The steering was very precise in the Pontiac, no play at all.

  52. Concerning comment by Hank Van Cleef re: 1. The shims under the reverse pawl bracket are not there for mechanical clearance, but for timing.
    To get to reverse from neutral, the rear band had to apply to stop the drum from spinning, but to get the reverse pawl to engage, the band had to start to release just as the pawl was entering the reverse unit teeth.
    I have 1948 Pontiac 8 that has correct Reverse gear backlash but exhibits high gear lever resistance to engage Rev from low. It also is very stiff to release from Rev back to low.
    When engine is off and no pressure the lever is smooth both directions. Does anyone know if this was normal in -48 or a fix relating to this complaint?
    Love reading your posts, which I found trying to find info on the above query.

    1. Bob,

      I’m not a repair technician, so I’m really not qualified to tell you how to repair your transmission. However, I had a look through the U.S. Army repair manual for the military Hydra-Matic and its troubleshooting guide suggests that the dilemma you’re experiencing might be the result of oil pressure that’s a bit too low at idle — either because the front pump isn’t up to spec or because all the pressure isn’t making it to the rear servo and reverse anchor piston. The fact that the resistance isn’t present with the engine off would seem to imply a hydraulic issue rather than a mechanical one like a worn or broken linkage.

      1. Thank you Aaron. I will advise the outcome of your suggestion.

        1. Aside from checking the pump and pressure regulator shuttle, the other suggestion the Army manual had was to make sure the valve body and its end plate aren’t loose. Of course, you don’t want to overtighten them either!

          (Again, I’m not a mechanic and I haven’t worked on Hydra-Matics myself — this is just what’s suggested in TM 9-1727C (Technical Manual: Ordnance Maintenance: Hydra-Matic Transmission and Propeller Shafts for Light Tanks M5, M5A1, and 75-mm Howitzer Motor Carriage M8), which you could certainly find a copy of online if you were so inclined. However, there are some differences between the military unit and the civilian automotive transmissions, so I would strongly recommend referring to the appropriate Pontiac repair manual shop manual supplement, especially for details like torque specifications and operating pressures!)

          1. Aaron I started by removing 1 of 3 reverse blocker shims to see what would happen. Surprisingly the problem was worse. I then made an additional shim and assembled with 4 total. The result was an improvement from original(3)
            This surprised me, but does confirm what Hank states earlier that the shims are for timing. Additional to this the line pressure is 95lbs @ 1000rpm, 80@ 600rpm. I also lowered the idle speed after the changes were made & this cleaned up the problem altogether.
            Note: this trans is rebuilt and in excellent internal condition.
            On another matter, I rebuilt one in a 1962 Rolls Royce 20 yrs ago (Silver Shadow if memory serves, with mechanical brake servo). It had absolutely no shift feel after rebuild and worried me that it would not last too long.
            The original throttle valve spring had been changed to a hair spring which I changed to something that looked like one shown in Motor Manual disassemble view.
            The resultant shift was beautiful, just discernible and nothing else.
            The owner reported back after he had the car in a concourse show that the judges knocked the car because the shift was not as per RollRoyce “feel” (none)
            Thanks again for your reply
            regards Bob

  53. Hi Aarron,

    Do you know which product is the first commercial automatic transmission that utilizes the combination of one-way clutch and low and reverse clutch? I tried to search for it on the internet but failed. Thank you!

    1. Do you mean like in a Ford C-6, where you have a Simpson gearset with a clutch instead of a band to hold the rear drum in both directions (while the one-way clutch prevents reverse rotation)? I’m not sure. (The first production Simpson gear automatic was TorqueFlite, but TorqueFlite and early Ford Cruise-O-Matic have a low/reverse band, not a clutch.) You might want to look into Philip Gott’s book Changing Gears, published in 1991, which is an exhaustive survey of automotive transmission development to that point.

  54. I drove a ’56 Buick Century for years, and in that time had to have transmission repairs twice. I habitually started off in L, then shifted to D at about 25 mph in normal street driving. The shift thump was not pleasant, but acceleration was much more satisfactory. I learned that the result of that habit was accelerated wear of the Drive piston lip seal. From standstill in Drive with such wear, the front pump couldn’t supply enough pressure to firmly engage the drive clutch, so slippage was terrible. Starting off in low became mandatory. It engaged firmly, and after the car got under way, the rear pump provided enough extra oil flow to overcome the leakage and raise the pressure sufficiently to keep Drive firmly engaged at street speeds.

    I also learned in the transmission shop that habitual drag racing (not me) using the Low range in a Dynaflow, often resulted in mechanical breakage. I suppose the mechanic was referring to the planetary gear cage.

    A friend once told me of a situation in a ’50s single coupling Hydramatic, where the car couldn’t get moving from a standstill, but with a push from another vehicle, would propel itself strongly after it was moving fast enough that the front planetary would have shifted out. As long as the car didn’t come to a standstill, this worked, the car could run at highway speed, and got the family out of Mexico where the failure occurred. I found in later years that this also worked with a dual coupling Hydramatic that had the same symptom (failed sprag clutch in that case, I suppose).

    As a final thought, I think the drivabiity of the single coupling Hydramatic could have been improved a great deal if they had incorporated part-throttle downshift. Regardless of wishful thinking, they’re still my favorite. Can’t help loving their low slippage.

    1. Aside from whatever seal wear you experienced, Dynaflow actually had variable pump pressure depending on manual gear selection. Line pressure was basically doubled in Low and Reverse to provide additional holding power for the bands.

  55. Thanks Aaron for this great work you do. The old Dual Coupling Hydro was my favorite all time transmission. Back in 1962 Pontiac had a performance reputation both in drag racing and stock car racing. Back then Fireball Roberts was still driving Pontiacs so as a teenager I was really enthralled. One guy’s dad had a ’59 Star Chief and two others had 61s, one a Star Chief and one a Catalina and they would all three fly. That summer my dad bought his first air conditioned car. He bought . 1962 Catalina. But when I took it out my buddies all ran off and left me. I ran it through the quarter at a whopping 80 MPH. My buddies were getting more like 90 MPH. What the heck I’m thinking? This thing’s a dog. It would roll top end wise and damn near hide the needle on the Speedometer. But for acceleration? My sister’s Renault Dalphine would give it a run. At least off the line. Through the next couple months I did a little research, stone age style, by reading everything from Hot Rod Magazine to various motor manuals. Then I stumbled on something. I was reading in a Chilton’s motor manual where for 1962 Pontiac’s Roto Hydramatic was in the Catalina and Grand Prix but the Star Chief and Bonneville still had the old 4 speed Hydro.

    Then when I took the car out I confirmed it only had three forward gears. I went riding with my buddy Marvin in his dad’s 61 Catlina. It had 4 forward speeds. Also, the Roto Hydro didn’t have a real positive lock up. It wouldn’t turn a tire on ice. Yet Marvin’s 61 would absolutely smoke the tires. When I made all this discovery I told my dad he needed to trade that POS for a Star Chief. But he wasn’t into speed.

    Also, I had a friend name Jack whose mother ordered her a new Cadillac Convertible every two years. His dad was the officer in charge of the local Coast Guard Station at that time. His mother had a 1963 Red Convertible with white top. That thing would fly. It would do an amazing 95 MPH in the quarter (Speedometer indication only, but still) and blow the doors on some of the fastest cars in town. It had the Dual Coupling 4 Speed Hydro, of course. Then the next year, which was late ’64, his mom ordered a new 1965 model with Turbo 400 Trans. It was nice and I think it had a few more horse power, but the ’63 would blow its doors off. Those two Cadillacs were the same model, even color, and close in weight and the 65 had more power but the 63 was faster through the quarter. Why? I believe it was the Transmission. Detroit doesn’t always change things for the better. I think the old Dual Coupling Hydramatic was the best transmission GM ever made.

    Thanks, Mike

  56. I read your very fine article because I told a friend of mine that I thought Cadillac had Hyromatic drive in 1941. He said no. I thought your article could give me the answer.Your article “sorta” mentioned a ’41 with Hyromatic in passing. So is it definite that ’41 Cadillac had Hydromatic drive?
    P.S I have a ’48 Caddy with Hyromatic and have had a ’41 Cadillac with standard shift. But I THINK I remember seeing a ’41 with Hyromatic.

    1. Hydra-Matic was an extra-cost option on 1941 Cadillacs, so some buyers got it and some did not. Roughly one-third of ’41 Cadillacs had Hydra-Matic and the rest had standard shift.

  57. What are the proper front and rear clutch pack clearances? I can’t find a specification anywhere.

    1. I’m afraid I can’t give technical advice except to strongly recommend that you look for a Hydra-Matic shop manual that covers the specific model and year of your car. (I wouldn’t assume that the clearances would be identical from model to model, since the clutches themselves aren’t necessarily the same.)

  58. Speaking of pre-war GM or in this case Chevrolet, does any information exist on an experimental overhead value V8 designed by British born engineer Alex Taub in the early-1930s that was (perhaps too hastily) rejected by Chevrolet?

    1. Well, a patent search finds some interesting work by Taub on related areas, including exhaust valve cooling (US1727621A) and combustion chamber design (US1757399A, which describes a four-cylinder iteration, and US2214941A, which describes an intake-valve cooling scheme). None of them is specifically for a V-8, but I imagine that in these, you can see some of what he may have been thinking in design ethos.

      Interestingly, after he left GM, he designed a horizontally opposed engine layout building on some of those concepts (US2506250A). That patent describes an F-head layout that he thought could be cast in aluminum and applied to H-4, H-6, H-8, or H-12 engines!

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