The Stovebolt Site Forums The Shop Area Engine Shop ID this rear axle

I am doing more detective work on my 54 GMC, which from what I found is not all 54 GMC anymore. For example it now has a Chevy 261, SM420 4-spd, 3/4-ton drive shaft assembly. Now I am trying to ID the rear-end. The original would have had a torque tube set up, but mine is now an open set up. The casting number on the carrier casting is 3707340. I did a web search and found someone else listing this as being in their 1957 3100 Chevy truck. Can anyone confirm this? I have taken it apart and I could get some pictures if neccessary. Thanks.

Jim

'68 parts manual shows that casting number as a carrier from '55-'62 1/2-tons (excl. posi). Also used in '63 4WD 1/2-tons and '57-'59 4WD front axles.

So it is definitely a 1/2-ton Chevy rear-end then, not a GMC. Well it looks like my entire drive train is not original. Is there any thing note worthy about this rear-end good or bad? One thing I noticed when I had it out of the truck is when I turned the pinion shaft to check the ratio I was able to reach over and stop the axle from turning on both sides one at a time. This makes no sense to me. I would think the the gears would be coupled and if I held one axle I should be able to stop the pinion shaft from turning. The one thing I was not able to do, since I do not have three hands is to see if while I was turning the pinion shaft and holding on axle if the other axle was now coupled. Can anyone explain this to me? Thanks.

Jim

Jim,
Your axle is fine, they are supposed to do that. It is part of the way non-posi differentials work. (Think about any time you got a vehicle stuck in mud or snow and watched just one wheel spin.) The reason for this is to allow one wheel to turn faster or slower than the other in turns, hence the name "differential." (The inside wheel travels a shorter distance than the outside wheel in a turn.) Also note that the grand majority of these years' 1/2 ton axles were 3.90:1 gearing.
Steve O

I did read that these axles were 3.90:1. My truck really seems to be winding out at 55 MPH, so I have been thinking about a gear upgrade or an axle swap. I have checked out a few of the vendor sites and they all seem to only offer a 3.38:1 replacement gears. Do you think this ratio change will allow my truck to cruise comfortably at 60-65 MPH?

yes the 3.38 ratio will give you much better hiway speed at lower RPM and still do fine at slow speeds - and it's about the only ratio you'll find to fit - other option is to put in an OD trans

Bill

I have an sm420 w/ 3:90 but it doesn't really get loud until 65-70mph. I can go 55-60 all day and get 14-17mpg. I have 30" tires - maybe that's the difference. I'm currently installing a gear vendors OD, because I want my truck to have the best of both worlds. It's expensive though, I had to save money for 2 years to get it. I'm shooting for comfy & quiet driving at 65 and 20mpg. P.S. I'm also getting 30" radials on 16" wheels w/stock centers.

You might want to look a little harder at exactly what year that truck is... GMC didn't do things quite the same as everyone else and the difference between a '54 and a '55 1st is a little vague.

While it may have a swapped in rear axle from a later truck, it could also be a '55 1st manufactured and titled in '54.

How could one tell the difference betwee the 55 1st and a 54 GMC? I pretty sure my truck is a 54 GMC in which the entire drive train was switched over to Chevy.

Jim