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Sm420 Parts Interchange Between Years?

Discussion in 'Early CJ5 and CJ6 Tech' started by chipdom, Jul 13, 2020.

  1. Jul 13, 2020
    chipdom

    chipdom Member

    06811
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    I purchased 3 transmissions which the seller claimed, 1 is good, 1 needs rebuild, and 1 for parts. 2 have the ribs on the passenger side with one having the reverse switch hole in the top cover, but both of these have case problems, one has a repaired broken mounting hole to bellhousing and the other has a nasty repaired crack. The usable one, after decoding the serial number seems to be a 1956. My question is, which parts are interchangeable? I would like to make a useable tranny out of the 3, and have the option of the reverse switch. Any and all information, opinions and suggestions will be greatly appreciated.
    Thanks in advance.
     
  2. Jul 13, 2020
    47v6

    47v6 junk wrecker! 2023 Sponsor 2022 Sponsor

    USA
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    From my experience, the SM420 has a couple different synchros. One is not really available. The newer ones used a seal in the front bearing retainer, while the older ones did not. I'm thinking the cutoff years for that was late 50s.

    I would take all 3 apart and see for yourself what is the same and make a good one. You don't really know who did what to them.

    The pins on the shift tower wear out. It make it hard to find gears but they are pretty easy to fix. I turned down a bolt and threaded the cover where the riveted ones wore out and were loose. Fixed it.
     
  3. Jul 14, 2020
    boopiejones

    boopiejones I can’t drive 55

    California east bay
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    I can’t answer your compatibility questions, but I wouldn’t worry too much about the reverse switch as you can pretty easily make your own. Just need to bend a piece of metal, drill a hole in it for the switch and attach to the transmission with a hose clamp... 86627FA1-BF63-4E30-A358-E323374813EA.jpeg
     
    Fireball likes this.
  4. Jul 14, 2020
    nickmil

    nickmil In mothballs.

    Happy Valley, OR
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    Sep 23, 2002
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    The reverse switch indicates a 1967 transmission, or at least shifter top. The guts should interchange between cases but as 47v6 said there are differences in the internal parts themselves. I've personally seen 3 different versions. I would suggest disassembling the one with the good case first, evaluate all the parts, and hopefully you can use any parts needed from the other two. I would suggest replacing all bearings and seals while you have it apart.
     
  5. Jul 14, 2020
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    Yes, parts will interchange between the 3 versions. The last version (64-67) has the aluminum synchro in the first gear for second. Those are not as good as the earlier brass ones. The ideal sm420 has all brass synchros, two ribbed case and the very late shifter top with the reverse light switch built in.

    I have 6 sm420s and of different eras.
     
  6. Jul 16, 2020
    chipdom

    chipdom Member

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    Thanks for the info. What I really need to know is if the 67 top cover is interchangeable to the 56?
     
  7. Jul 16, 2020
    nickmil

    nickmil In mothballs.

    Happy Valley, OR
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    Yes
     
  8. Jul 17, 2020
    chipdom

    chipdom Member

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    Just what I wanted to hear. Much thanks
     
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