Don’t Call It Hydra-Matic: The Rolls-Royce and Bentley Automatic Gearbox


From 1953 to 1978, British automaker Rolls-Royce manufactured its own versions of the General Motors Hydra-Matic transmission for use in Rolls-Royce and Bentley luxury cars. One version was very similar to the Hydra-Matic used in American cars and trucks, but Rolls-Royce later created an improved aluminum-case version with modifications to provide smoother shifting. This version was offered in some versions of the Rolls-Royce Silver Shadow I and Bentley T Series from 1966 to 1969. The older version remained in use in the big Phantom V and Phantom VI limousines until 1978.


In the autumn of 1965, Rolls-Royce unveiled two all-new models to succeed the Silver Cloud III and Bentley S Series: the Rolls-Royce Silver Shadow and closely related Bentley T Series. The new cars retained the 6,230 cc (380 cu. in.) aluminum V-8, but were otherwise thoroughly updated, now using monocoque construction (with front and rear subframes), fully independent suspension with hydraulic self-leveling (using technology licensed from Citroën), and four-wheel disc brakes.

Left-hand-drive cars also had a new transmission: the latest GM Turbo Hydra-Matic (a.k.a. TH400), shared with some contemporary full-size General Motors cars. Although Rolls-Royce and Bentley units had serial numbers beginning with “RR” (and, according to some sources, tighter production tolerances than the GM norm), these three-speed torque converter transmissions were manufactured by Hydra-Matic Division rather than in Crewe. Rolls-Royce literature generally describes this transmission as “GM 400,” but the factory workshop manuals also use the Turbo Hydra-Matic name.

1967 Rolls-Royce Silver Shadow (dark blue LHD) front 3q by RL GNZLZ (CC BY-SA 2.0)

Although more modern-looking than the Silver Cloud III it replaced, the Rolls-Royce Silver Shadow I was actually a bit smaller, stretching 203.5 inches (5,169 mm) on a 119.5-inch (3,035-mm) wheelbase. Standing 59.8 inches (1,517 mm) high unladen, the Silver Shadow was also 3.8 inches (95 mm) lower than its predecessor, although it remained tall and narrow compared to a contemporary Cadillac. Remarkably, this left-hand-drive Silver Shadow uses the same transmission as a contemporary Cadillac; Rolls-Royce made Turbo Hydra-Matic standard on all LHD Silver Shadow and T Series export models from launch. (Photo: “Rolls Royce Silver Shadow 1967” RL GNZLZ, which is licensed under a Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0) license)

Early right-hand-drive Silver Shadows and T Series Bentleys did not have the GM 400 transmission, instead continuing with the fluid coupling four-speed automatic. It would be natural to assume that this was a carryover of the transmission from the last Silver Ghost III and Bentley S Series, but in fact the four-speed transmission had undergone some substantial changes, creating the final new variation of the original single-coupling Hydra-Matic.

The first major change was a new case. Where earlier Hydra-Matic transmissions were primarily cast iron, the new unit had a cast aluminum case, including the bell housing, tail extension, and some internal castings. (Crewe-built transmissions already had an aluminum side cover and sump.) We haven’t found any figures suggesting what weight savings this change achieved, but we would guess something on the order of 60 to 70 lb (27 to 32 kg), offset somewhat by some of the other internal revisions.

A second important revision, shared with Rolls-Royce and Bentley GM 400 transmissions, was an electric shift actuator system, providing fingertip control of the various driving ranges. The actuator was an external add-on, mounted on the side of the tail extension. It used a double-wound electric motor to move the transmission selector shaft lever, with a solenoid-operated brake to hold the actuator securely in place after each selection. Apparently having noted the troublesome results of earlier American attempts at electric shift control, Rolls-Royce added an access port in the transmission tunnel through which a small tommy bar (described in the workshop manual as the “Get-You-Home Bar”) could be inserted to manually operate the selector level if the electrical system failed. The electric shift actuator also had a new R-N-4-3-2 shift pattern; on earlier cars with the four-speed automatic, reverse was below “2” at the bottom of the pattern.

1967 Rolls-Royce Silver Shadow drophead coupé (by H.J. Mulliner, Park Ward) interior and dashboard by Andrew Bone (CC BY 2.0)

Built in 1967, this Silver Shadow two-door drophead coupé (predecessor of the Corniche) has the four-speed automatic gearbox. A close look at the shift quadrant in the original photo reveals the R-N-4-3-2 shift pattern; cars with Turbo Hydra-Matic had a P-R-N-D-I-L pattern. (Photo: “Rolls-Royce Silver Shadow Drophead Coupé by H.J.Mulliner Park Ward (1967)” by Andrew Bone, which is licensed under a Creative Commons Attribution 2.0 Generic (CC BY 2.0) license)

The third and most significant change was in the transmission’s internal layout, which was revised to incorporate one of the changes GM had made for the Controlled Coupling Hydra-Matic: a one-way sprag clutch that took over the role of the rear brake band in “4” and “3” ranges. The effects of these changes are explained in more detail in the sidebar below, but the object was to smooth out the transmission’s occasionally harsh shifts.


Add a Comment
  1. 1. Thank you for new content.
    2. I appreciate the research undertaken to provide this new content. Entertaining and informative. Job well-done!
    3. I learned that Rolls switched to Chrysler-style lifters for their OHV V8 engine; they did prototype work with Buick lifters and had camshaft lobe failures. Not surprising–Buick lifters (Nailhead/Nailvalve era) had no crown, the Buick camshaft had no taper, and the Nailhead lifter bores were not offset on the lobes. The Buick Nailhead lifters were not intended to rotate in their bores. I don’t know of any other V-8 OHV engine that intentionally does not spin the lifters. Lifter spin reduces wear.

    1. The Chrysler lifter design is mentioned in the text, as it was another example of Rolls-Royce engineers considering a problem, deciding existing American technology offered the best solution, and then arranging to manufacture it in-house. It’s also clear that developing the V-8 involved a pretty steep learning curve in various respects.

  2. An excellent review. Thank you!

  3. Aaron, while automatic transmissions in luxury cars are not my jam, reading your wonderful scholarship is. Thank you!!!Jir

  4. Aaron, great to see new content here, and to the same high standard as ever. I wonder if RR’s decision to licence and build, rather than merely import was due to to government currency restrictions regarding Sterling and the US Dollar?

    1. I had the same thought, particularly when I saw that Rolls-Royce arranged to pay for the imported transmissions used in 1952–1953 cars in U.S. dollars. My understanding is that in that period, for a British company to pay for a transaction in dollars would have been only slightly less politically fraught than arranging to pay in bone marrow! However, Robotham’s account makes no mention of political considerations like that when he went to the U.S. in spring 1946.

      Also, if the rationale for licensed production was political, I have to think GM management would have been less puzzled. Wilson, surely, would have understood the impetus for local production due to import restrictions, unfavorable currency exchange rates (although it’s notable that the trip took place well before the subsequent devaluation of the sterling), or the costs of importation. The only explanation Robotham’s memoir offers for not wanting to import the transmissions rather than build them is that it was “unthinkable.”

    2. I drove a Rolls for a wealthy fellow in the early had the exact same radio/cassette player as my ‘86 Cheney S-10, albeit with gold plating.

      A nice ride. What I remember most was it was quiet, and there was little or no plastic in the interior

  5. Coming in a bit late..many years ago I read that when RR received the first batch of Hydra Matics, they were shipped dry (of course) and RR as was their procedure, decided to “improve” the finish of the internals parts by cycling walnut hulls through them. This caused problems (I think the walnut oil messed up the rubber seals) and GM told them to stop doing that.

    1. I’m just glad that anyone is still reading, having gone years without new content to speak of.

      With Hydra-Matic, it’s clear that Rolls-Royce made a variety of efforts to gild the lily, as it were, generally ending with the crew in Crewe having to concede that there was no good in fixing what wasn’t broken.

      The one really substantive change they made from a mechanical standpoint (prior to the aluminum-case version used in the early Silver Shadow) was eliminating the forced upshift from second to fourth in Low range. With the Dual-Range Hydra-Matic, GM had concluded that it was better not to let the driver override the automatic controls to the extent that it risked over-revving the engine; you could delay upshifts, but no further than the hydraulic controls would otherwise allow at WOT. Rolls-Royce apparently felt that if a driver manually selected “2,” they damned well wanted second gear and shifting up should be at their discretion. This might have improved performance a bit, because with the GM units, the automatic upshift in Low was to fourth, not third. Winding out in second and then shifting to “3” would probably give slightly better acceleration. How many Rolls-Royce drivers would have bothered, I have no idea, since it would seem likely to spill milady’s mustard.

  6. Echoing others’ comments, it’s great to see some new content and the older articles continue to be prime points of reference. A bit of a tangent but the mention of Robotham’s name being abbreviated to two letters in internal memos reminded me that when I joined the Rolls-Royce aero engine business in the mid ‘90s (which had been separated from the car company since 1971), the Derby part of the company was still using this convention to the great confusion of those of us who worked at other sites. I’m not sure when it died out, but not long after.

  7. Still reading!

  8. Another great read, and I thank you for it. Not that I’m in the market for one, but couldn’t R-R at least put at the column shift on the left of the column for RHD cars?

    And that steering wheel (styling wise) is more suited to a bus.

    1. And that steering wheel (styling wise) is more suited to a bus.

      Which wheel? The Silver Cloud wheel does seem rather bus-like, perhaps because until the Silver Cloud II in 1958, power steering was a pricey extra, and some allowance had to be made for the chauffeur on cars without power assist. (Cars with power steering did get a smaller wheel set closer to the dash, but the difference in diameter was about an inch, and so I think it would take a practiced eye to spot the difference.) The two-spoke wheel in the Silver Shadow doesn’t strike me as particularly bus-like, although I can’t say I find it particularly attractive or in keeping with the general ambiance. A detail, I guess, but at these prices, the details ought to be exquisite.

  9. I only discovered this site about a month ago and it is now my favorite auto site along with Mac’s Motor City. I really appreciate your in depth dives into various models and technology. I hope you continue to publish more as I can’t get enough!

  10. Thank you for a very informative amount of info on RollsRoyce/Hydramatic transmissions.
    My daughter is interested in a 1956 Armstrong Siddely fitted with automatic transmission,when I queried the seller is it automatic or preselector he said someone told him it was the same as RR used.
    That car is now a lot more interesting as a result of your information thank you.

Leave a Reply

Your email address will not be published. Required fields are marked *

Comments may be moderated. Submitting a comment signifies your acceptance of our Comment Policy — please read it first! You must be at least 18 to comment. PLEASE DON'T SUBMIT COPYRIGHTED CONTENT YOU AREN'T AUTHORIZED TO USE!