The Stovebolt Site Forums The Shop Area Engine Shop gmc 1/2 trans question

I just sold my 49 chev 3/4 ton and bought a 49 GMC 1/2 ton longbed. The truck is very similar. My new GMC has a 3 speed on the column. I want to put in an sm 420 4 speed. That is what I had in my 49 and liked it. Is it a direct bolt in?

I think it should be a straight replacement. An SM420 was a factory option for the GMC so it should go right in. My experience is with Chevies, though. With them, the 3-speed 1/2-ton had a 9 or 9 1/8" clutch. All other models have a 10 3/4" clutch, so that's something to consider. Do you have a Shop Manual for the GMC? I recommend you have one - very helpful. I think the extension for the torque-tube drive shaft is at the rear of your longbed Jimmy 1/2 ton. And I don't think the length of the trans are different, so the ball/socket joint on tail of trans should not be a problem. Of course the 4-speed trucks have a large removable floor panel/cover for the trans. You'll have to work around that and make a hole in the floor for the shifter. Good Luck. :p

You might try Old GMC Trucks website for more technical info about your pickup. Also a search of this forum of post relating to SM420 might yield more useful info. I also like this site for good info about this transmission. Good luck

Thanks for your response. The transmission is the same length so I assume it will just bolt up.

The 3/4 ton yoke on the sm420 is different than the 1/2 ton. The 3/4 ton version is beffier. The 3 speed yoke will not interchange. You will need to find a sm420 from a 1/2 ton to get the correct yoke.