Modern Transmission Behind an Early Oldsmobile V8 Engine

TH400 transmission


Can you tell me what would be involved in putting a turbo 400 trans behind a 1963 olds 394 cubic inch motor?


Most ‘63 Oldsmobile 394ci engines were backed up by the Roto Hydramatic Transmission, a lightweight transmission also known as the “Slim Jim”. Their problems included: a high-pressure design that made them prone to leaking, huge jumps between gear ranges, and were designed to use transmission fluids that contained whale oil, that are now banned today. Modern ATF fluids further degrade shift performance.

Making an upgrade to a TH400 transmission would certainly improve performance and reliability, although I would suggest going one step further and adding overdrive with the use of a 700r4 or 4L60 transmission. The original Slim Jim transmission is a relatively narrow transmission and upgrading to the TH400 may require some modifications to the transmission tunnel in your floor, A TH350 may be an easier fit and can be built to handle most anything you throw at it.

The next challenge is connecting the engine to the transmission. The 394 Olds engine has a deep skirt cast into the back of the block, making it difficult to join the two together. Fortunately our good friend Bob Bendtsen at Speed Gems ( has us covered with a great adapter kit to make the connection. Having actually installed one in a ’62 Oldsmobile, I can tell you it is a great kit and certainly got the job done. However, this entire conversion process is not a simple bolt in process and we’ll get into that next.

The next BIG hurdle to jump, is motor mounts… while you never mentioned the model of car your 394 called home. The 394 uses a center mount front motor mount and two mounts at the back of the engine to hold the engine from moving from side-to-side. Unfortunately the rear motor mounts cannot be used after the transmission conversion and new rear motor mounts will need to be fabricated. Additionally the transmission will require a rear mount on the tail shaft and a removable cross-member will need to be fabricated and installed. From my experience, this is all very doable however more than a little head scratching is involved.

Of course you’ll need to have a new driveshaft made to mate the TH400 transmission and the rear axle together, and if your going to keep the original shift lever on the column, it will need to be modified to work. Otherwise there are a host of aftermarket floor shifters that will get the job done.

If you do keep the original column shift the gear position indicator will need to be reworked as the “Slim Jim” transmission had a highly criticized P-N-D-S-L-R shift pattern, and the TH400 has the more standard P-R-N-D-S-L shift pattern. The TH400 also utilizes an electronic kick-down for the transmission; Lokar offers a kit to make this aspect of the conversion easier.

Bottom line: Can you mate a TH400 behind an Oldsmobile 394? Yes! Is it a good upgrade to make? Sure, although I would still lean toward adding overdrive as well. Is it easy to do? I guess it depends on your mechanical abilities, your fabrication skills and the tools and shop you have available. Would I do it? If I had my heart set on keeping the original 394, sure, I’ve done it. But have always second-guessed my decision to do so, as I could have more easily transplanted a 455ci Oldsmobile engine and TH400 transmission.

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13 Responses to “Modern Transmission Behind an Early Oldsmobile V8 Engine”

  1. gary arcure

    Would it have been easier to convert a '59 or '60 Jetaway to the '63 Oldsmobile 394?

  2. Dale

    I have a Muncie M22 4speed in my 63 Pontiac Strato Chief. I want to convert it over to a 700R4 automatic and floor shifter. Could you advise me of the best vintage trans to use and the accessories such as flex plate, converter, and controller or cable. Is there a video available.

  3. Paul Poirier

    Why not install a 200 4R of course built to handle the torque .

  4. Raymond Bixby

    I like this. Have a 455 w/TH400 in my 71 CUTLASS SX

  5. Fred MacDonald

    A buddy of mine ended up with a 1962 Olds 88 with a bad engine. We installed a slightly warmed over 394 and the tan 4-door hard top was literally a street sleeper! It would more than give my 350 4-speed Camaro a hard time! If you put that Olds to the floor it shifted into high gear at 105 mph! Yes, that transmission was a little rougher shifting than a turbo 350 or 400, but we never had any trouble with it.

  6. wayne bailey

    I wanted to keep my 1961 394 tri power motor but didn't want the old trans. I wanted to upgrade to an overdrive so after a lot of measuring I decided to adapt a 700r4 to my engine. I first made an adapter for the flywheel to accept a flex plate with the starter ring removed. I then cut the front edge of the 700r4 bell back and then welded a homemade adapter plate to it so as to allow the converter to go deeper into the extended skirt of the engine. In the bell housing, there were bungs welded in to accommodate allen head or tweve point bolts which gives attaching points for the trans to the engine. The side trans mounts were used but modified by cutting a corner off each one. The rear engine starter plate can still be used with the assembly, no trimming necessary. after putting trans to the engine, it all fits nicely and the gap on the converter is correct. I haven't install the complete assembly into the car but the trans should fit similar to the SlimJim since the ears and some of the 700r4 trans bell has been removed. You will need lathe work performed for the flywheel adapter and a tig or an aluminum spool gun for the welding, micrometer and some hand tools.

  7. Miguel Ledezma

    Me gustaría saber si tiene el kit de conversión de la transmisión,tengo un 1962 Oldsmobile dynamic 88 con motor 6.5 lts?? Porfavor si me puedes ayudar

  8. Steve Cudd

    I’m going to Ross Racing Engines to get a TH 400 with their adapter from tranny to engine. He says no tunnel mods required. He supplies the torque converter and everything that is needed.

  9. Eric

    I am building a 1963 Ford Falcon gasser. I have a 394 olds engine all refreshed, but no tyranny. What would be a good trans for it without moving the rear motor mount already transferred to the Falcon?

  10. chad walker

    I'm trying to find out what a 62 olds sky rocket is worth please? It runs well. I also have the transmission it needs some new parts. It shifted ok. But it leaked. Thank you