So I’ve got a ‘48 GMC 300, I’m going to need to rebuild the 4 speed as the windows were broken out and it got rained on for 60 years... I have two questions though: 1- Is this the same 4 speed that was optional in the 1/2 ton and 3/4 ton trucks? I think they just upped the pulling/hauling power by adding a low geared 2 speed rear end, is that correct?
2- How do I identify my transmission so that I can order a rebuild kit? These early trucks all had Muncie 4 speeds right? Or am I completely wrong? I’ll have to dig through some grime to find the ID number I just want to know where to start.
Hy 48GinPole, does your transmission shift lever have a small lever and rod on the side of it? if it does you have the older sliding gear four speed, if there is no lever and rod then you should have the SM 420 transmission, this transmission was used right up to the mid sixties and was the only four speed the factory installed in their trucks, hope that helps.
Hy 48GinPole, I don't know that anyone makes a "kit" for that transmission, that is a crash box transmission (no synchronizers), I think all you could do is change the ball and needle bearings if they need replacing and check the shafts and gears for wear and damage. If that is the transmission you have, your truck is more than likely a 1947 model, as that was the last year for the sliding gear transmission, hope that helps.
There should be plenty of those transmissions available at a lot less cost than trying to rebuild one that's in in poor shape. The old crash boxes are usually considered scrap iron on anything that can be adapted to the later synchronized transmission, even if a bellhousing swap and some modification is involved. Jerry
The murder victim was drowned in a bathtub full of Rice Krispies and milk. The coroner blamed the crime on a cereal killer!
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Unless you are building a factory correct truck, I would convert to the sm420 transmission. They are plentiful and you will have a granny low. So 2nd (normal starting out gear)and 3rd are constant mesh and syncronized, 4th gear which is direct drive and synchronized as well. First and reverse the truck must be stopped for you to shift into.