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SM420 identification, advice

Discussion in '40- & 55-Series Tech' started by pablo, Jun 17, 2003.

  1. pablo


    Likes Received:
    Mar 5, 2002
    I saw a couple of transmissions at the yard yesterday that seem to work. First off, what is the spline count for the imput and output shaft of this gear box? Also, is there a transmission that was used on GM vehicles that may be confused with the SM420 during the same era? Why is there no SM420 marks on the casting. I found some with 390 and Pinheads begins with 381. However, the SM465 has a 465 on it. One gear box in particular that came out of a Chevy Apache had a drum brake assembly on the rear and a funny shifter looking thing on the passenger side. I am not sure if it actuated the drumb brake. Would one of these transmissions work. Also there was one that had a frozen imput shaft bearing It probably has considerable rust in it as well. Should I just stay away from it all together?
    Thanks Pablo

    By the way, I found a pretty stripped down FJ-55 there if anyone needs anything.
  2. 73lndcrsr

    73lndcrsr SILVER Star

    Likes Received:
    May 28, 2002
    I found one this past weekend that looks mimilar to what you described. The distinguishing feature as I understand it is the bulge on the passenger side. Mine has the parking brake. It just un-bolts.

    I think I would stay away from one if it was significantly rusted inside.

    Just my opinion.
  3. Pin_Head


    Likes Received:
    Feb 6, 2002
    OC, CA
    SM420s out of 30 and 50 series (1 and 2 ton) GM trucks had the parking brake drum on the rear and the brake lever on the passenger side. These will work well after you remove the brake assembly. You really can't miss identify a sm420. It is found on GM trucks from 47-67 and it has a large bulge for the reverse gear on the passenger side with a PTO ocver on the driver side. The casting marks vary on sm420s. Some begin with a 3 and others begin with a 5 and this indicates a split in some minor variations in the internal parts, such as the synchro rngs, that are not interchangeable. The year of hte change is approx. 1958.