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55 Plymouth O/D trans?

Discussion in 'The Hokey Ass Message Board' started by NOT_SO_FAST, May 6, 2012.

    Joined: Oct 25, 2008
    Posts: 228

    from IL

    One of the 'yards I frequent has a '55 Plymouth with the 230 flathead six and some type of overdrive manual transmission. I'm thinking of trying get the engine and trans out of it if the price is right. I have a '51 Coronet and a '55 Dodge truck,both with the same engine,would it be a worthy upgrade for either vehicle,or just a waste of time?
  2. Hnstray
    Joined: Aug 23, 2009
    Posts: 11,967

    from Quincy, IL

    I've done a few of these and believe it is well worth doing. I had a '51 Coronet Diplomat with Gyromatic and removed that and replaced with Mopar 3 spd/OD and it made the car a pleasure to drive on the highway. The engine rpm came way down, the OD ratio is about .70, so cruising at 60/65 was a breeze without a screaming engine. I also did a 3 spd non OD swap on a '49 Dodge that originally was Fluid Drive and the process is the same as described, just no OD.

    Did the same thing on my '49 Plymouth with the same satisfactory results.

    All that said, may have a problem regarding you '51 Coronet. Depending on what transmission you currently have. As you surely know, both the Fluid Drive and Gyromatic have a fluid coupling as well as a manual clutch. When I swapped out my Gyro I removed everything inside the bellhousing from the crankshaft back. The bellhousing on those cars is very long to contain the fluid coupling and manual clutch.

    I installed a manual flywheel, and a '53/'54 Dodge 3 speed w/OD trans which has the much longer input shaft required when the there is no fluid coupling. Also, the longer '53/'54 throwout bearing collar is required for the same reason. The original '51 clutch linkage and shift rods work ok.

    If you have a fully manual 3 speed now, no fluid coupling, then you would have the long input shaft I am referring to. It could be swapped into the '55 OD trans and you are in business. If you have Fluid Drive or Gyromatic........(buzzer sound here) will need the long input shaft from the correct manual trans. Not so easy to find these days.

    One other alternative would be to use the '55 Plymouth bellhousing, flywheel, clutch. trans etc. But, the crossmember supporting the longer bellhousing you now have would have to be relocated forward and your driveshaft would need lengthening.

    Your '55 Dodge truck would likely be a 'bolt in' on the back of the engine. I am not quite so familiar with the back of the truck trans, if it is the same length as the passenger car box, then your driveshaft should fit, as well as all linkages. You need only install the OD control cable and wiring harness and you are off to the races.

    Edit: just noticed your pickup is "C3"...what is it? 1/2, 3/4 or ?? my comments about it were based on the assumption it is a 1/2 ton....oops !

    Last edited: May 7, 2012
  3. RAR1947
    Joined: Jul 18, 2009
    Posts: 24

    from Minnesota

    Can't provide as much info as Ray stated above, but will say that I think its worth it. I've got a 47 Plymouth with an early 50's overdrive unit and can say its probably one of the best upgrades for the drive-ability of the car. Cruising down the road at highway speed is very relaxing for the old flathead. Very satisfied with my setup and would do it again.
  4. its worth grabbing the OD.

  5. it is TOTALLY a worthwhile endeavor, but, in the event you don't care for it, or decide not to do the swap, there are MANY flathead six fans who would jump at the availability of the parts, and would likely be willing to pay you handsomely for your time.
  6. what they said....the little mopar trannies interchange from about 1940 to 1958 or so....the little flathead 6 was discontinued in 1959...please get the OD trans for somebody...

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