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.
Hi all when I rebuilt my 4 speed in my 42 truck I just replaced the bearing and seals the rest of the box was still good I made my own gaskets. Heavy box for the size of it Cheers Justin
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
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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.
Last edited by sstock; 07/22/20183:05 PM.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Yeah, that looks like the square gear first in the sm420. Bet that trans howls like a pack of wolves with those straight cut gears.
1953 Chevrolet 3100 261 cu inch, sm420, 3.55 rear, torque tube still,omaha orange, still 6 volt, RPO green glass, side carrier spare, all done In the DITY Gallery Video of the 261 running
1964 GMC 1000 305 Big Block V6, sm420, the next cab off restoration
Bet that trans howls like a pack of wolves with those straight cut gears.
Not necessarily. My Model A with straight cut gears wasn't that noisy. If they get worn, yes, they'd be noisy, but if in decent shape, shouldn't be bad. A bit noisier than spiral cut gears, but not bad.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
A bit of info on the later gearbox.... The '48 onwards used a Muncie SM420 which bolts up to bell housings with the standard Muncie Chevy bolt pattern. To put one behind a 350 you need an early '56 Chevy bell housing with the open bottom to access the lower threaded bolt holes in the transmission.
With a little gentle practice one can double clutch a 420 , NP-540 or even a Dana 18 or 20 transfer case while moving, saves clutch wear with a load on a hill.
Ed
'37 GMC T-18 w/ DD 4-53T, RTO-610, 6231 aux., '95 GMC running gear, full disc brakes, power steering, 22.5 wheels and tires. '47 GMC 1 ton w/ 302, NP-540, 4wd, full width Blazer front axle. '54 GMC 630 w/ 503 gasser, 5 speed, ex fire truck, shortened WB 4', install 8' bed. '55 GMC 370 w/270, 420 4 speed, grain, dump bed truck from ND. Works OK.
