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Technical Muncie SM420 into 1941 Coupe

Discussion in 'The Hokey Ass Message Board' started by Jrom460, Aug 17, 2022.

  1. Jrom460
    Joined: Aug 21, 2009
    Posts: 2

    Jrom460
    Member
    from Eindhoven

    Hello everyone!

    I am having issues with my three on the tree in my 1941 Chevrolet Coupe. It pops out of 3rd gear when driving on the freeway. I learned this is probably due to worn out / tapered gears and can be solved (temporarily) with shims. But since this is only a temporar fix, I am looking for another transmission. Im located in Europe so parts availability is limited. I tried but just cannot find a 3spd tranny. But Ive stumbled upon several 4spd Muncie SM420, they even had the torque tube connection.

    Would this fit into my coupe?

    P.S.
    I know this has a floor shift and a useless granny 1st gear, Im OK with that, just asking if it would fit mechanically...

    Thanks!
    Jrom
     
  2. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,590

    Truckedup
    Member

    Second through forth are widely spaced but it can be shifted fairly fast..Extremely robust, still in demand for offroad 400 hp rock climbers
    It's been awhile, but I belive it will bolt in to replace the torque tube 3 speed ....compare lengths
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 52,380

    squirrel
    Member

    check the bolt pattern on the front of the transmission, you might find that the rear surface of the car bellhousing is much smaller than the bolt pattern on the front of the truck transmission. Trucks used a different bellhousing, can you get one with a bellhousing?
     
  4. Truckedup
    Joined: Jul 25, 2006
    Posts: 4,590

    Truckedup
    Member

    , Corrected...
     
    Last edited: Aug 17, 2022

  5. 302GMC
    Joined: Dec 15, 2005
    Posts: 7,362

    302GMC
    Member
    from Idaho

    Horrible choice .... I did that to a '51 fastback when I was still drinking.
    The bellhousing has to be changed to a truck item, which is too deep, so the engine has to be lowered to clear firewall or just bash it in. .
    The mounts for the bell have to be fabbed.
    By the time all this is done, the bottom of the trans is 3'' from the highway ...

    Why not rebuild your 3 speed - the high gear jump can be cured by eliminating the excess end play in the top train caused by wear in the snap ring groove for the rear bearing. Shim the clutch gear back, or add a shim behind 2nd gear ...
     
    Last edited: Aug 17, 2022
    Truckdoctor Andy and squirrel like this.
  6. Mr48chev
    Joined: Dec 28, 2007
    Posts: 31,845

    Mr48chev
    ALLIANCE MEMBER

    302 GMC pretty well nailed it. an SM420 will not bolt up to a 54 or earleir Car bellhousing nor will it bolt up to a 47 or earler Truck bellhousing.
    All SM-420 transmissions have what is known as the "Muncie" bolt pattern. This being a 1951 Chevy truck bellhousing that had an SM-420 bolted to it.
    This is the truck bellhousing that others referred to.
    Even if you got the trans in place you still have to change the driveshaft and rear axle as even a closed drive SM420 may not bolt directly to the early torque tube.
    IMG_6071 (3).JPG
     
  7. HEATHEN
    Joined: Nov 22, 2005
    Posts: 7,880

    HEATHEN
    Member
    from SIDNEY, NY

    I've seen several early Chevrolet three speeds with an excessively worn 2-3 shift fork that never really pushes the gear all the way in to start with. Have you removed the side cover to check out anything yet?
     
  8. 24riverview
    Joined: Jan 13, 2008
    Posts: 998

    24riverview
    Member

    My son's 54 Chevy had the same issue, we found a bunch of slop in the rear main shaft bearing. You could pick up the bearing by the outer race and the inner race would drop close to .125". Better used bearing cured it.
     
  9. Jrom460
    Joined: Aug 21, 2009
    Posts: 2

    Jrom460
    Member
    from Eindhoven

    Thank you very much to you all!
    I think I will try to rebuild the tranny. I already put new main bearings in but I will now add shims as well. The shift forks are good actually. I will also check the shift mechanism for play and if It needs some further adjustments.
    This will be the fastest and cheapest option I think.
    In the meantime I'll check for sm420 lengths to see if it would fit. My idea was, instead of using a truck bellhousing, I could make an adapter plate between car bellhousing and sm420....
    Thanks again!!!
    Jrom
     

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