The Stovebolt Site Forums The Shop Area Engine Shop Older SM420? transmissions

I just ordered a Novak adapter to put a SM420 into my '64 Jeep. I have a good one out of a "53 step van, but I also have other 4-speeds out of older trucks. One was a "49 GMC for sure. Are these shorter tranmissions also SM420s? They are about 1.5 inch shorter and have a trigger to get into reverse. The gear ratios seem the same, but the Novak tech wasn't familar with these transmissions and didn't know if they would work. The shorter 1.5 inch may make a difference in mounting to the cross member in the Jeep. I'll try the adapter on these when I get it, but was wondering if there was a website or other information sources to read about these older ones. Back to one 420 that I have for sure. I need to get the shifting rod out and reverse it. Pushing down and turning doesn't seem to work. In fact, I can't push it down at all. Help on how to do this would be appreciated. (I know a Jeep is a four letter word, but it sort of looks like a Chevy cab and has a cool grill. Besides, it has a chevy engine in now)

It's OK talking about SM420 with Jeeps for it was a Jeep forum which I learned how to install a backup light switch in a SM420.

Don't know why you cannot remove the shifter after pushing down and twisting the lever collar. It should happen with little effort.

All of the Chev and GMC 4 speeds with a trigger reverse lockout that I have seen were pre 48 non syncro units.

Makes sense. Without the synchro rings, the transmission could be shorter. Are older transmissions still SM420s? Other than the synchros and shifter, is the rest the same? I still can't get the shifter out of my "real" 420. It's spring loaded in the upwards direction but not downwards. Coming out of a '53, it may be an older one. Also, I read info on how the one casting number is a date code. The letter being a month, then day, then year. Mine is F642 on the 420 which doesn't add up. I know.. just use it and be happy, but part of the fun is the research. Also, I have pieces and parts from a '46 1-ton. Would this truck have an open driveshaft? I need to know which trans to keep for this project. Any help on these topics will be appreciated.

I just tore my SM420 apart to inspect it. The cap has to be pushed in about 1/8 of an inch and then rotated counter clockwise. The configuration holding the shifter in resembles that of an 1156 light bulb. Make sure you have it in neutral while doing this. Hope this helps, and good luck with the tear down.

That trigger is exactly what was previously said.
It is a lockout for reverse and came in the Art Deco/Liberty series trucks Early 47 and older
Late 47 thru 67 were the standard sm420s that everyone is so used to
Go to the Novaks site and look up the history,it tells all including that there was no backup lite switch port until the last year or two of production
Regards