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Sm420 Reverse Light Switch

Discussion in 'Builds and Fabricators Forum' started by FinoCJ, Mar 12, 2018.

  1. Mar 12, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member 2020 Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
    Messages:
    2,515
    Is there a solution to installing a back-up light switch to an sm420? I think the last year of the 420 (1967?) may have come with one, but this would be more about retrofitting one to a unit that did not come with one.
     
    piffey263 likes this.
  2. Mar 12, 2018
    n6ifp

    n6ifp Member 2019 Sponsor

    San Bruno/...
    Joined:
    Sep 15, 2004
    Messages:
    206
    I found the reverse shift and drilled the forward end of the cover and added a switch. You do have to take a good look at the forward end of the shift rod, because it has a depression in it. You need to forward of that to make the switch stay on. I had picture and measurement,but can't find my pics. I will continue to look.
     
  3. Mar 13, 2018
    Danefraz

    Danefraz Well-Known Member 2019 Sponsor

    Chico CA
    Joined:
    Aug 11, 2012
    Messages:
    1,963
    I did a similar thing with the T-98 I installed. I found the reverse shift rod in the top, drilled and tapped, added a reverse switch.

    tricky part is finding a short-version and then setting the depth correctly so the detent ball in the switch is triggered by the rod.

    d
     
  4. Mar 22, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member 2020 Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
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    2,515
    the reason I am asking:
    Muncie sm420 4 speed transmission

    Heading through here on my way up to Montana again and wondering if I should check this out...obviously I can't see inside from the pics, but it looks like someone put some time and effort into the outside which is a good sign. What is a reasonable price?
    Also, I noticed from looking at the sm420, it has a bit of a belly that would hang down pretty far down and cause clearance issues? Or is that just an optical illusion. My next big expense (planned) on the jeep is an eaton locker in the rear, and this would probably have to sit around for awhile. Also, the expense of the adapters is certainly a factor as well as driveshafts etc...and I would have to change my barrel gear for the my OD (which is also specific to the D18 TLs).
     
    Last edited: Mar 23, 2018
  5. Mar 23, 2018
    47v6

    47v6 junk wrecker! 2020 Sponsor

    Washington DC.
    Joined:
    Oct 25, 2006
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    4,723
    I have the sm420. Its a pre 1953 model and it does not have a seal for the input, but a reverse spiral to keep gear oil in. If you park it downhill it can leak some gear oil. These transmissions are old, but parts are available. I rebuilt mine an did it cheap, but should just have bought the kit from Novak. It shifts slow and 1st gear is useless on the street. The transmissions were made to get a big load of dirt or grain or whatever going with a relatively low powered engine. The novak adapter is about 500 bucks. Its a well made part, but not cheap. The transmission does hang down, but not below the crossmember that you may have to move. I am not sure if you will need to shorten the rear driveshaft.

    You will probably need a new pilot bushing and a chevy clutch friction disk. You will also have to figure out clutch linkages.

    As is the story for al the "upgrades" I did on my jeep, I did it because it was cheaper than rebuilding the stuff I had. My sm420 was 75 bucks, but needed a new 4th gear synchro. Double clutch it and it was fine.

    I like my SM420 well enough, but It seems that people are equally as happy with T-18s. They are old and to do it again I would probably go automatic...:whistle:
     
  6. Mar 23, 2018
    gunner

    gunner Member

    Washington state...
    Joined:
    Dec 4, 2012
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    596
    I recently picked up a couple 420s, one was already adapted to a Dana 20 with a twin shift kit. I don't know if the adapter is Novak or AA, but the whole setup was $200. I'm not sure how good the 420 is as it had had some water in it from a PO, but it wasn't worn out, at least. The other one didn't have water damage but looked like it had more miles on it. I got it as a backup.

    Point is, you'd think in your area, you'd be able to find a 420 already adapted to a D18 or 20 for a whole lot less than going and buying the adapter and individual components. I don't know how far I'd go out of my way for a 420- I figure both mine were nearly free.

    By the numbers on the case of the 420 you are looking at (37XXXX), it's a later tranny like both of mine. Mine were made in 64 and 66. To confirm, try to get pics of the build date and the pass side bulge to see if the reinforcing ribs are on the bulge.

    How much to give for the CL one? It's been advertised for a month. Considering you may be going out of your way somewhat, I'd start pretty low.
     
  7. Mar 23, 2018
    FinoCJ

    FinoCJ 1970 CJ5 Staff Member 2020 Sponsor

    Denver, CO
    Joined:
    Jul 18, 2013
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    2,515
    Its been posted for a couple months - I keep checking as I drive through Ashton a few times every winter (as I will be on Sat). All that helps mean I don't have to overpay for it...most of the ones I see around here (Colo) look like they need a lot of love...was wondering if this one might not need as much love. Budget is a bit tight right now - and I have to kind of make a decision as to whether to work towards the 4-speed or towards the locker...I don't really need a CR of 107 (with a sm420), but currently at 48, and would really like to have more (60-70 would probaby work). hmmm...decisions decisions decisions....
     

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