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4 Speed questions #1272230
Thu Jul 12 2018 01:16 PM
Thu Jul 12 2018 01:16 PM
4
47GinPole  Offline OP
New Guy
Joined: Jun 2018
Posts: 17
Arizona
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.

Thanks in advance!


Logan
2010 Silverado 2500HD
1947 GMC 300
Re: 4 Speed questions [Re: 47GinPole] #1272234
Thu Jul 12 2018 01:41 PM
Thu Jul 12 2018 01:41 PM
3
3B  Offline
Master Gabster
Joined: Nov 2005
Posts: 5,196
B. C. Canada
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.

Re: 4 Speed questions [Re: 47GinPole] #1272263
Thu Jul 12 2018 05:37 PM
Thu Jul 12 2018 05:37 PM
4
47GinPole  Offline OP
New Guy
Joined: Jun 2018
Posts: 17
Arizona
It does have the the small lever, what does that do? Ive never heard of that. Is it going to be hard to find a rebuild kit?


Logan
2010 Silverado 2500HD
1947 GMC 300
Re: 4 Speed questions [Re: 47GinPole] #1272268
Thu Jul 12 2018 06:53 PM
Thu Jul 12 2018 06:53 PM
3
3B  Offline
Master Gabster
Joined: Nov 2005
Posts: 5,196
B. C. Canada
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.

Re: 4 Speed questions [Re: 47GinPole] #1272301
Fri Jul 13 2018 03:36 AM
Fri Jul 13 2018 03:36 AM
R
Rich'sToys  Offline
Shop Shark
Joined: Jul 2004
Posts: 3,219
Janesville, Mn
The small lever is a reverse lock out. You need to lift the lever to put it into reverse, which would be to the left, then back toward the seat.


Rich

'47 Loadmaster


Politicians and Lawyers have twisted everything in this country into knots that we can never unravel.


I would rather live my life as if there is a God and die to find out there isn't, than live my life as if there isn't and die to find out there is.




Re: 4 Speed questions [Re: 47GinPole] #1273338
Sun Jul 22 2018 04:10 AM
Sun Jul 22 2018 04:10 AM
A
Aus42blackout  Offline
Shop Shark
Joined: Jan 2010
Posts: 69
Rockhampton, Queensland, Austr...
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

Re: 4 Speed questions [Re: 47GinPole] #1273357
Sun Jul 22 2018 02:00 PM
Sun Jul 22 2018 02:00 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 17,009
Dellrose, TN
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!
Re: 4 Speed questions [Re: 47GinPole] #1273366
Sun Jul 22 2018 03:02 PM
Sun Jul 22 2018 03:02 PM
S
sstock  Offline
Shop Shark
Joined: Jan 2013
Posts: 856
Gig Harbor, WA
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; Sun Jul 22 2018 03:05 PM.

1953 Chevrolet 3100
Re: 4 Speed questions [Re: 47GinPole] #1273454
Mon Jul 23 2018 03:34 AM
Mon Jul 23 2018 03:34 AM
D
Dusty  Offline
Shop Shark
Joined: Feb 2001
Posts: 981
Canterbury , NewZealand
sstock , the 4spd crunchy box has a wonderful "granny" low smile

Look at the size of that gear .... I took the top off my box to fix the pins in the gear lever .

Attached Files
Feb.2014.031.jpg (81 downloads)
Re: 4 Speed questions [Re: 47GinPole] #1273468
Mon Jul 23 2018 04:53 AM
Mon Jul 23 2018 04:53 AM
S
sstock  Offline
Shop Shark
Joined: Jan 2013
Posts: 856
Gig Harbor, WA
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
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