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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8 Responses to “Modern Transmission Behind an Early Oldsmobile V8 Engine”
  1. Yailenis

    The reason you don’t want to sanbdlast the whole thing is that the sand gets everywhere and there’s no way in heck you can get all the sand out of the nooks and crannies. So, what happens is that the sand will settle in little pockets and when the car gets wet, the sand and water will mix and settle in that same spot and eventually cause rust. Rust is like cancer for a car and it’ll rust from the inside out-it’ll ruin your body and paint. If UR going to build something to last, do it right!

  2. John Cooper
    John Cooper

    Having just graduated high school, I purchased my first car a1964 Oldsmobile 98 during the summer before attending community college in the fall for an automotive program. The car didn’t run when I bought it and was the subject of some repairs such as brakes and exhaust in my automotive classes. One thing always stood out about that car to me was the incredible 394 engine and the hurkey jurkey shifting transmission. My shop instructor said the transmission used in my car was known to shift like that and that it would probably be okay. He never really explained what the issue was, but he was right about it being okay. So I have always wondered exactly what the shifting issue was and the explanation in your column makes perfect sense. It seemed to be shifting to another gear that was like shifting from 2nd gear to 4th gear in a manual. The gear ratio must have been too far apart causing a lag and jerky shifting. It only took 40 years to get that question answered. LOL! Thanks for your insight.

  3. Dean

    I am now the proud owner of a 62 Starfire. The only problem I have is that the car is in 100 pieces. It’s an original Napa Ca. car that the previous owner was restoring when he passed away. So much is complete but now comes the tough stuff like the dash and wiring harness. If anyone knows a shop that understands oldsmobiles here in Norcal or Reno NV. I’d appreciate some help. I have the 394 rebuilt and the original transmission but I’m not sure which one. Floor shifter, automatic.

    • Larry

      First, get a FACTORY shop manual. Next, join the ANTIQUE Oldsmobile Club of America and ask the same question on their forum.

  4. chad walker
    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

  5. 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?

    • Customer Service
      Customer Service

      Hello Eric,

      This is for members to our online community. We do have a $3.00 annual entrance offer if you are interested? This would include access to expert advice (like this), plus discounts, hours of Premium videos, etc. Below is the link to join if you are interested:

      Classic Car Restoration Club Video Membership

  6. Steve Cudd
    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.