Dynaflow, Turboglide, Roto Hydramatic, and Other Early GM Automatics

The Hydra-Matic, GM’s first fully automatic transmission, was a great success, inspiring a host of rivals — including some within General Motors itself. This week, we look at the origins of Dynaflow and Powerglide, the ambitious but ill-fated Turboglide and Triple Turbine, the later controlled coupling Hydra-Matic and Roto Hydramatic, and more.

1951 Buick Super Riviera Dynaflow badge

TORQUE CONVERTER DRIVE

As we saw in our first installment, Oldsmobile’s original Hydra-Matic, introduced in 1939, was the world’s first really successful fully automatic transmission. By 1952, GM had produced more than two million Hydra-Matics, which were used by Oldsmobile, Cadillac, Pontiac, and a variety of other automakers, ranging from Kaiser-Frazer to the short-lived Muntz Jet. Hydra-Matic was standard on Cadillacs by the early fifties and went into more than 80% of all Oldsmobiles and Pontiacs.

Notably missing from Hydra-Matic’s list of users were GM’s other automotive divisions, Chevrolet and Buick. Buick’s absence was particularly curious considering that the division had actually manufactured Hydra-Matic’s short-lived semi-automatic predecessor, the 1937-1939 Automatic Safety Transmission. Although corporate management had browbeaten Buick general manager Harlow Curtice and chief engineer Charles Chayne into offering the semi-automatic transmission in 1938, Buick’s engineers had never liked it and they liked Hydra-Matic even less.

There were several reasons for Buick’s disdain for Hydra-Matic, among them divisional pride. Some Buick engineers still resented the cancellation of the “Roller,” a friction-drive automatic they’d developed in the early thirties, which they felt was a superior transmission. They were also displeased with Hydra-Matic’s endemically jerky shifts. As we discussed in last week’s article, each of the H-M’s gear changes was mechanically complicated, involving a coordinated application and release of brake bands and clutch packs. Shifts were always firm, and if the transmission was not in perfect adjustment or if the bands were worn, it changed gears with a jolt. On Oldsmobiles and Cadillacs, which used open driveshafts, this could be mildly annoying. Buick and Chevrolet, however, used torque tube drive, which combined the driveshaft and axle into a single, rigid unit carried by the rear springs and connected to the transmission via a single universal joint. With a torque tube, each shift sent a pronounced thump through the frame and body, which Curtice and Chayne, in particular, considered unacceptable.

2006 Porsche torque converter cutaway BerndB
A cutaway model of a modern Porsche torque converter. The lower set of blades are the blades of the stator, which redirects the flow of oil returning from the turbine to the impeller. The springs visible near the center of the image are part of the lockup clutch, which mechanically locks the turbine to the impeller for greater efficiency at cruising speeds. Packard used a lockup torque converter on its Ultramatic transmission, but lockup converters were not common on automotive transmissions until the late seventies. ( © 2006 BerndB; used under a Creative Commons Attribution-ShareAlike 3.0 license)

Engineer Oliver K. Kelley, who had been part of Earl Thompson’s Transmission Development Group (which had led the development of the Hydra-Matic), had similar reservations. Hydra-Matic worked, but it was not a particularly elegant design, being both complex and expensive to build. In the fall of 1939, Kelley and his colleague, George Smith, wrote a letter to Victor Olsen, general manager of the new Detroit Transmisssion Division, suggesting that it would make sense for the corporation to develop several distinct automatic transmissions, optimized for the needs of the various automotive divisions. Olsen agreed and Kelley and Smith returned to the corporate Engineering staff to begin work on a simpler, cheaper automatic transmission, originally intended for Chevrolet.

Kelley and Smith’s thinking soon turned away from fluid couplings (as used by the H-M) to torque converters. A torque converter is a type of fluid coupling, with an additional reaction member (a stator) that allows it to multiply engine torque at certain speeds, much like mechanical reduction gears. The torque converter was not new — it was first patented in 1905, at the same time as the ‘plain’ fluid coupling — and it was common in marine and industrial applications. Inventor Oscar Banker had patented a torque converter automatic design in the early thirties that had seen some use in trains and buses and GMC began using Spicer torque converter transmissions (designed by Lysholm) in its Yellow Cab coaches in 1938, but converters had yet to catch on for automobiles.

The appeal of the torque converter for Oliver Kelley was that it offered the possibility of torque multiplication without the use of conventional gears. An torque converter automatic could offer a useful range of multiplication with only a single planetary gearset — and eventually, perhaps, with no gears at all. Such a transmission would be both simpler and cheaper than Hydra-Matic, better suited for inexpensive cars like Chevrolet.

The ramp-up of military production just before America’s entry into the second world war turned the Transmission Development Group’s thinking away from passenger cars to a different application: armored fighting vehicles. Cadillac had already started adapting Hydra-Matic for use in tanks, but its torque multiplication was adequate only for lighter tanks and its gear changes still resulted in brief reductions in speed, something that could be fatal in combat. The torque converter drive offered the possibility of greater torque multiplication as well as an uninterrupted flow of power.

The first tangible result of the Transmission Development Group’s labors was Buick’s M-18 Hellcat tank destroyer, which used a modified Allison TorqMaster bus transmission with a torque converter replacing the clutch. The Hellcat proved very successful, validating many of Kelley’s ideas. It set the stage for torque converter transmissions for GM’s postwar passenger cars.

M18 Hellcat Tank Destroyer
An M-18 Hellcat tank destroyer. Designed and manufactured by Buick, the Hellcat massed just under 20 tons, was capable of more than 50 mph (80 km/h), and was armed with a 76mm (3-inch) gun, giving it the ability to destroy the latest German Panzers. Its limitation was extremely thin armor — it relied on speed for survival. Buick built about 2,500 Hellcats, some of which remained in service for many years after the war’s end. (Photo © 2007 “Dammit”; used under a Creative Commons Attribution-ShareAlike 2.5 Netherlands license)

BUICK DYNAFLOW AND CHEVROLET POWERGLIDE

As the war wound down, Kelley’s group resumed work on automotive torque converter transmissions. Although their original thinking had revolved around Chevrolet, in mid-1945, Oliver Kelley demonstrated a prototype to Charlie Chayne, who was so impressed that he talked Harlow Curtice into test-driving it personally. Curtice subsequently ordered Buick’s own engineers, led by Rudolf Gorsky, to refine the corporate engineers’ ideas into a production version for Buick.

1949 Buick Roadmaster Dynaflow badge
Dynaflow was initially a $206 option, available only on Roadmasters; it became standard on Roadmasters and optional on other models in 1949. Specials and Supers with Dynaflow had a higher compression ratio and slightly more power than standard-shift cars to make up for the transmission’s lower efficiency.

Buick’s new transmission, which went on sale in January 1948 under the name Dynaflow, took the torque converter drive concept a step further than the wartime Hellcat had. Although it used two planetary gearsets, providing two forward ratios plus reverse, Dynaflow did not shift at all in normal driving. Instead, it relied on the torque multiplication of its complex, five-element torque converter. To expand its useful range of operating speeds, the converter used two impellers and two stators, each with a different blade pitch. At stall, it provided a maximum torque multiplication of 2.25:1, not far from the 2.39:1 first gear of Buick’s standard manual gearbox. As engine speed increased, the converter multiplication gradually faded; at cruising speed, both stators freewheeled and the converter acted like a conventional fluid coupling. In effect, Dynaflow was what we would now call a continuously variable transmission, although unlike modern belt/chain/roller-driven CVTs, its operation was purely hydraulic.

Dynaflow provided unparalleled smoothness with no discernible gear changes. The consequence was lethargic low-speed throttle response. Unlike a mechanical reduction gear, the torque converter produced useful multiplication only at certain speeds. If you caught it outside that optimum range, stabbing the throttle produced more churning than forward motion, even with the 276 lb-ft (373 N-m) of Buick’s biggest straight-eight engine. Slippage was also high, which resulted in heavy fuel consumption. Buick tried to compensate for the latter by using a taller (lower numerical) axle ratio with Dynaflow, but that did nothing for performance.

1949 Buick Roadmaster front 3q
The 1949 Roadmaster was distinguished from lesser Buicks by its longer 126-inch (3,200mm) wheelbase (compared to 121 inches/3,073mm for other models); both figures were down 3 inches (76 mm) from 1948. The Roadmaster came standard with a big 320 cu. in. (5,247 cc) eight, which made 150 gross horsepower (112 kW); starting in 1949, Dynaflow was standard as well.

There was a ready cure for Dynaflow’s mediocre acceleration: manually selecting Low range added a 1.82:1 mechanical gear ratio to the converter’s multiplication. Low could be held to about 50 mph (81 km/h), providing much snappier takeoffs and stronger passing response. Unfortunately, the transmission had no way to shift automatically into high; the driver had to move the selector back to Drive and doing that too frequently tended to damage the clutches and bands. Buick’s owner’s manual cautiously described the low gear as “emergency low.”

Despite those shortcomings, Dynaflow was well suited to the character of postwar Buicks, which emphasized refinement and comfort over performance or road manners. The average Buick buyer was not terribly concerned with fuel economy and welcomed Dynaflow’s lazy smoothness. It was too bad that Buick no longer offered limousines; Dynaflow lent itself admirably to a processional pace.

1952 Chevrolet Bel Air Powerglide badge
Early Powerglide-equipped Chevrolets came with a bored-and-stroked 236 cu. in. (3,859 cc) version of Chevy’s Stovebolt Six with 105 hp (78 kW) to the standard car’s 92 (69 kW). It was essentially Chevy’s truck engine, but with hydraulic lifters.

Dynaflow was followed in 1950 by Chevrolet’s Powerglide. Powerglide was a variation on the Dynaflow theme: conceptually similar, but differing in detail. Its greatest novelty was a second, smaller fluid coupling, mounted inside of the inner pump vanes of the five-element torque converter; the smaller coupling provided engine braking in downhill coasting.

Powerglide suffered the same limitations as the Buick transmission, which were more pronounced with Chevrolet’s six than with Buick’s eight. Powerglide-equipped Chevys came with a bigger, more powerful engine, but the new automatic added more than seven seconds to 0-60 mph (0-97 km/h) times and resulted in less-than-frugal fuel economy.

1952 Chevrolet Bel Air Powerglide quadrant
The early Powerglide had the same PNDLR shift quadrant as the early Dynaflow; its operation was similar to the Buick transmission in most respects, although its small second fluid coupling provided additional engine braking on the overrun.

HYDRA-MATIC VERSUS DYNAFLOW

We may take it as a sign of GM’s considerable wealth and resources that it entered the fifties with three distinct automatic transmissions while most of its competitors were still struggling to develop even one. Until the debut of Chrysler’s two-speed PowerFlite in 1953, the only other automaker to develop its own automatic was Packard, which had introduced its torque converter Ultramatic in 1949. (GM, not convinced that the venerable independent had the resources for such a feat, later sued Packard, claiming Ultramatic’s torque converter design infringed on Buick’s patent.) Both Studebaker and Ford turned to Borg-Warner to develop their first automatics, while most of the other independents either bought Hydra-Matic or went without.

In a later era, GM’s corporate management would undoubtedly have pushed for standardization, but in the early fifties, General Motors controlled nearly half of the largest automotive market in the world and allowing the divisions to remain independent and competitive was an affordable indulgence. Just as important was the fact that GM had invested considerable capital in development and tooling for Hydra-Matic, Dynaflow, and Powerglide, which the corporation was not about to casually discard.

1950 Oldsmobile 88 quadrant
In the late forties and early fifties, Hydra-Matic was an expensive option on most of the cars that offered it, but it went into more than four-fifths of all Cadillacs, Oldsmobiles, and Pontiacs. This 1950 Oldsmobile Eighty-Eight has the earlier Hydra-Matic, with only a single drive range; the Dual-Range version replaced it in 1952.

Moreover, the divisions’ customers were developing strong feelings about the pros and cons of each transmission, which sometimes resembled the rivalry between competing sports franchises. Hydra-Matic fans extolled its efficiency and crispness; Dynaflow supporters proclaimed the virtues of that transmission’s seamless action, and derided Hydra-Matic’s lurching shifts. Even the chief engineers of Oldsmobile and Buick exchanged good-natured jibes about the comparative merits of “Dyna-Slush” and “Hydra-Jerk,” although the latter camp had to swallow their pride for a while in 1953, when the destruction of the Livonia Hydra-Matic plant briefly forced Olds and Cadillac to use Dynaflow. The irony is that both transmissions emerged from the the same group in the corporate Engineering Staff; browsing through GM’s transmission-related patents from this period reveals many common names and evidence of similar design methodology.

Other manufacturers quickly settled on a compromise — two- or three-speed planetary gearsets combined with a torque converter. Chevrolet went that route in 1953, reengineering Powerglide to start in low and shift automatically to high; Packard adapted Ultramatic to do the same late in the 1954 model year. Instead, GM’s engineering staff spent the next decade refining the Dynaflow and Hydra-Matic concepts.

43 Comments

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  1. Hey,how come you can yack all day long about this ones gearset setup,or that ones turbine combination,but no illustrations???
    Just because you can picture the entire mechanical world with words doesn’t mean the rest of humanity can.
    Pictures Please!!!!

    1. Um, no “Thank you for an awesome article and site?”

      There is an illustration of a Turboglide and it’s hardly fair to expect Aaron to write an great article about the development of the automatic AND delve into all the technical details. He does to a degree, but that’s not the overwhelming emphasis of the site, as far as I understand it.

      How about Googling “Turboglide,” “Dynaflow” or “Powerglide?”

    2. Well, the problem is that unlike the cars themselves, the transmissions don’t have devoted fan clubs who like to show off their cunning internal components at shows. I can’t legally scan pages from the shop manuals, so I’m limited to what diagrams I can create myself.

      I’m not a technical illustrator by any means, and some of these designs are very complex — I don’t know that I would be able to competently render (even in a simplified way) how the pieces interconnect. I had hoped to create one for the controlled-coupling Hydra-Matic, but after staring at the shop manual for days, I mainly understood why GM decided it was too complex and too expensive to manufacture. If I have time and feel inspired, I may create some additional diagrams later and add them back in, although I’m not going to be putting any professional designers out of work…

      1. There’s a site here that has a diagram of an overhaul of the controlled coupling hydra-matic. I can really see why GM wanted to get way from this design. Although today’s ZF 8 and 9 speeds are probably worse, but then half of the world industry is sharing the development costs for these.

        1. …And yet, they were damn near indestructible. We had a ’58 Pontiac that took a lot of punishment in the snow, yet worked without any issues, other than a small oil leak, until I had to sell it in late 1964.If I remember correctly, it was cast iron and weighed around 225 lbs.

          1. The ’58 edition weighed about 240 lb. GM was able to trim about 10-11 lb for 1960 by slimming down the case a bit.

  2. In the photo of the Hydra-Matic shift quadrant in the ’50 Olds 88, is that an aftermarket turn signal unit? If so, it’s a reminder of how times have changed! I understand that at that time, a heater was an option on many cars.

    1. I believe turn signals were standard on Oldsmobiles by 1949, at least on DeLuxe models. I’d need to find somebody with an Olds dealer book from that period to know for sure, but my information suggests they were standard fit.

      Pretty much everything [i]else[/i] was at least technically optional at that point, including oil filters, wheel covers, hood ornaments, windshield washers, and (at least until after the war) reversing lamps. Heaters didn’t become standard even on Cadillacs until almost the mid-fifties, and they weren’t standard on cheap cars for another decade after that. Very few cars were built without a lot of these items, but they weren’t included in the list price for many years.

  3. At least they did not charge extra for chrome after the war.

    I remember seeing a ’50s car ad that mentioned the [i]reverse[/i] gear was an optional extra. On the other hand many cars (particularly British) came with leather seats only because it was cheaper than vinal.

    1. I don’t know of any cars that late that didn’t come with a reverse [i]gear[/i], although reversing [i]lamps[/i] were still extra on many inexpensive cars at that point. Turn signals, as well.

    2. Just as well they didn’t charge extra for chrome.
      The ’58 Buicks & Olds would have cost a small country to buy.

      Back on topic, thank you once again for an
      entertaining read.

      Cheers,
      Chris

      1. Well, in essence, they did charge extra for the chrome, though fortunately not by the pound. On most cars of that era the amount of brightwork was tied to the trim level, and naturally the higher the trim level, the higher the price. Beyond that, there were often extra-cost dress-up packages (either factory- or dealer-installed) that primarily consisted of additional chrome trim. Such things didn’t really disappear from American options lists until the rise of Japanese-style tiered equipment packaging quite a few years later.

  4. Thanks for a great website and particularly for the GM transmissions articles. Every article I’ve read has been complete, accurate, and very interesting.

  5. Thank you for the automatic transmission article(s) on GM. Finally, someone has accurately chronicled the myriad development story for us.
    Your site is a valuable and entertaining resource – keep up the great work!

  6. This brought back some memories – I remember when I first got my license driving my Dad’s ’65 Olds F-85 with Jetaway and those 1-2 shifts at about 70mph if you held your foot in it. I have a question – I have an childhood memory of an early 50′s vehicle ( think it was a Chevy ) with a “Torque-Glide” logo on the trunk lid instead of “Power-Glide”, but that can’t be right, can it?

    1. Chrysler had a number of semi-automatics in that period with a variety of bizarre names: Gyro-Torque, Fluid Torque Drive, Fluid-matic, Fluid-Drive, and Plymouth’s Hy-Drive. Maybe it was one of those?

  7. Someone left an anonymous message saying:

    [i]under the heading “Twin Turbine Dynaflow,” the second paragraph contains the following statement:

    “The primary turbine’s output shaft now drove the planet carrier of a planetary gearset, adding its mechanical gear reduction to the multiplication provided by the stator;”

    However, it seems to me that if the planet carrier were driven, then the planetary gearset would provide a step-up, or overdrive, ratio, rather than gear reduction. For gear reduction, the power would have to be taken out of the planet carrier and put in on either the sun gear with the internal gear locked, or put in on the internal gear with the sun gear locked.

    See what you think. [/i]

    There was definitely something amiss — I checked and found that I had garbled the explanation of the torque converter layout from the 1955 Buick owner’s manual. The answer is that the primary turbine drives the ring gear; within the primary turbine’s operating speed range, the sun gear is locked, and the pinions and planet carrier are driven at reduced speed. I’ve amended the text accordingly. Thanks for the note!

  8. anyone have a diagram of the dual path? It stopped shifting from low into second and I found a spring in the bottom of the pan. Where does it go?

    1. Sorry, I’m not qualified to give repair advice. You might try seeing if your local library has a service manual for it — I was able to find a copy of the Pontiac dual-coupling Hydra-Matic shop manual that way.

  9. [quote=steve dill]anyone have a diagram of the dual path? It stopped shifting from low into second and I found a spring in the bottom of the pan. Where does it go?[/quote]If you could provide a picture of the spring, I could look it up in my various manuals and give you an answer.

  10. I have a 62 Buick,Skylark,with the dualpath Tranny.the trans is in direct drive,only goes foward,no neautral,park orreverce,is thier a fix for this.

  11. this article was great. It answered my question as to why the 52 Super I just inherited doesn’t shift….that would be because it isn’t made to shift automatically….I read a blog online saying
    1952 Buick – the slowest car I ever loved….so true!

    1. Well, there’s an old saying to the effect that you can make anything fit if you have a big enough hammer. I honestly don’t know how much trouble would be involved in interchanging them, but since they were never designed to be used behind the same engines or in the same cars, I imagine it would take some work.

      At one time, Buick Nailhead engines were popular with drag racers, so if you were asking this question in, say, 1964, there might have been aftermarket kits to mate an older Buick V-8 with a beefed-up Powerglide. (Some drag racers used Powerglide because it consumed relatively little power and they didn’t need a lower first gear.) Today, I suspect you’d have more luck finding some way to put in a Turbo Hydramatic. I’ve never looked, though.

      This is a question that would probably be best put to a performance transmission manufacturer or a shop that specializes in parts for older transmissions.

  12. chevy had 2 auto transmissions in 61and62 1 was a turbo glide the other was –glide that changed by fluid. there was no gears in the trans. on the gear selector was P R D G G was for grade as going up a hill. what was the name of that trans?

    1. The two transmissions were Powerglide and Turboglide. Powerglide was the familiar two-speed-plus-torque-converter Chevrolet automatic, while the transmission you’re thinking of was Turboglide, which is described in the text.

      The G position was for Grade Retarder. It was intended not for climbing hills, but for descending them; it was supposed to mimic the effect of engine braking, of which the Turboglide otherwise didn’t allow very much. The Grade Retarder was not useful for acceleration or hill climbing, although some people had problems because they assumed it worked like the Low position on Powerglide, which was definitely not the case!

  13. Re read this as a refresher on the development of the automatic. Thank you again. Your site is an invaluable resource and I cannot thank you enough for doing what you do.

  14. Thank you for your clear and concise explanation of Dynaflow, and how it differs from the other two GM automatics. As we were a "Buick family," the innate superiority of Dynaflow was never a question; it was an article of faith. I remember the feelings of incredulity and betrayal I felt when I was told for the first time that Dynaflow was "Just Powerglide with a different name," and that Hydramatic was obviously better, because Olds and Cadillac used it. You have restored my faith in Dynaflow.

  15. We have recently inherited a 53 Roadmaster. I think it is an early model serial #26854377 because the 322 nailhead has a weighted pully instead of a rubber loaded harmonic balancer. The Dynaflow is now in the transmission shop and we are finding puzzles. According to the shop manuals the 53 should be the new twin turbine with only 1 pump and one stator. This trans has the words "twin turbine" cast into the bellhousing. But inside it has 2 pumps and 2 stators. Do we have a transitional factory job or a trans shop hybrid? Was the change made to save money (fewer parts) or to improve performance? Will our new Roady rise and fly?

  16. Hi can any body help me
    I have a 1958 Buick Road Master fitted with a Dynaflow Flight Pitch
    gear box can any one tell me where i can get spares for the gear box
    and will ship them to England

  17. Just wanted to say this is a great article. I started out looking to find the difference between the hydra-matic dual range and the strato-flight and wound up learning a lot more.

  18. The article refers to the Hydramatic’s jerkiness. Actually, many Hydramatics were so smooth that you could not even feel the shift; you could just hear the drop in engine speed. I remember in 1959 riding in a 1949 Lincoln with Hydramatic; it accelerated quickly and so smoothly that I could not feel the shifts. The same was true with some other cars with Hydramatic in which I rode, including a 1950 Pontiac, and those were all before GM introduced the Hydramatic with the second (controlled) fluid clutch in 1956. On the other hand, I rode in a 1953 Cadillac with had very firm shifts.

    The downshift resulting from flooring the accelerator were another matter; they were always accompanied by a mechanical clunk.

    1. The issue with the original Hydra-Matic was that because its shifts were mechanically complex (particularly between second and third, which was the most complicated sequence), its smoothness depended a great deal on how well the bands were adjusted, the condition of the transmission fluid, and other maintenance- and condition-related factors. If everything was perfectly adjusted, it would be quite acceptably smooth (particularly by the fifties, by which time GM had made a lot of minor refinements). If not, it would throw off the shift timing just enough to make the shift jerky, albeit not necessarily enough to really impair the transmission’s function. I suspect a lot of owners who complained to their dealers or mechanics were told, "Ehh, they all do that."

      Even some of the engineers who originally designed the Hydra-Matic thought it was too complicated for its own good, which is why they subsequently got into the torque converter automatics, which didn’t shift at all. The original Dynaflow was very much the antithesis of the Hydra-Matic in a lot of these respects.

    2. My experience with Hydromatic cars was that they were fairly smooth in shifting. PowerGlide cars had a very pronounced jerk when shifting. When my city purchased GM buses in the sixties, the Hydromatic was very rough when shifting with an easily heard lowering in engine sound as speed increased.

      1. The difficulty with making blanket statements in this area is that each of these transmissions was around for a long time in several quite distinct versions, not all of which felt or acted the same.

        Early Powerglide cars did not shift at all in normal driving — like the Buick Dynaflow, the original Powerglide took off on the converter alone in drive. (Powerglide was revised in 1953 to start in first and shift automatically to second.) So, early Powerglides (or Dynaflow) were smoother than even a well-adjusted early Hydra-Matic, albeit not especially quick or efficient. After that, there were early (iron-case) and later (aluminum-case) Powerglide transmissions, tuned in different ways for different engines.

        Similarly, the early (1940 to 1955) and late (1956-1964 dual-coupling) Hydra-Matics were significantly different mechanically — albeit still related — and felt quite different.

        So, while it may sound pedantic, it’s important to qualify statements like, "X was smoother/rougher than Y."

  19. I’ve heard a story about the Hydra-Matic, as follows:

    Supposedly Rolls-Royce acquired a Hydra-Matic for evaluation. They liked it but thought one particular part had too rough a finish. When they fabricated a smoother-finished version of the part and incorporated it into the reassembled Hydra-Matic, the transmission didn’t work. True, or urban legend?

    1. I’ve heard that story in regard to the Turbo Hydramatic (not the original), which Rolls-Royce also built. The way I’ve heard it is more that they tightened up the tolerances, which didn’t necessarily work out well. I don’t know if it’s true or not, but it’s not implausible. There’s an analogy to be made with pistols, where getting everything "tuned" to tight tolerances improves accuracy, but makes the action less tolerant of dirt or debris. (This is why police and military sidearms are not built like target pistols.)

  20. Thank you for this very complete summary. I have been curious about these transmissions for quite some time, and this is quite helpful. Your research is impressive, as is the writing.

  21. The main problem with reliability of the Slim Jim was the weakness of the front oil pump cover; they cracked. An improved pump with webbing on the cover was designed to replace failed units. RHM 375 Model 10′s made at Willow Run ceased in 1962. The THM 350 signalled the beginning of a long slide toward mediocrity by GM.

    1. I have to wonder if the Roto Hydra-Matic’s various weaknesses, including the propensity for leaks and the issue you describe, were exacerbated by the very high operating pressures. As mentioned, the RHM’s operating pressures were substantially higher than the earlier dual-coupling HM’s, which is a lot of added stress to put on what was still fundamentally an adaptation of the earlier transmission.

      I’m not sure how your last statement follows. The THM350, which didn’t arrive until five years or so after the RHM expired, was effectively a replacement for the Powerglide and Super Turbine two-speed automatics, and in that sense were an improvement in most respects. (There have been some harsh criticisms of the later TH200, but that’s a different story.) Since most rivals had long since offered three-speed automatics for most engines, the TH350 was also arguably overdue. It wasn’t quite as heavy-duty as the TH400, but it wasn’t designed to be, trading off some torque capacity for lighter internals and lower power consumption.

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