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10-Bolt GM Differential, Info Needed

6K views 7 replies 4 participants last post by  jmbo-52 
#1 · (Edited)
My 36' Chevy Master Deluxe, has a 10-bolt GM differential under it,
Drum brakes
10 Bolt,
on the forward side of the passenger side axle housing, there are the following stamped into it ' P.E. G304 1' this information is about midway between the Pinion housing and the drum brake backplate, passenger side.

On the passenger side of the pinion housing, there are these characters: GM23 which appear UPSIDE Down .

On the driver side of the pinion housing, there are these characters. I 2 9 8 along with a date clock, see picture DSC0093.jpg

On the pinion housing, closest to the Pinion nut itself, is these number:
1 2 5 4 3 3 3

The bottom of the casting for the pinion ring housing has squared off corners on both bottom left and bottom right.

The housing vent is located on the top of the pinion housing on the passenger side of the car, closest to where the axle housing meets the pinion housing.

The Pinion nut itself looks to be 1.1/4"

I will attempt to upload pictures,
Can someone Identify the possible year of the casting, the possible gear ratio (consider as if it has never been changed up or down).
If it is posi, limited, or open, and from what type of vehicle it may have come out of.

This is the rear differential that was under the car when I purchased it, previous owner deceased, therefore no answers from him.

Thanks in advance.

Liberty
 

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#2 ·
Casting #'s off the centersection are pretty much worthless as far as finding any information on GM rears.

From your description of the squared off corners and the 1-1/4" pinion nut points to it being an 8.5" 10-bolt. Since it doesn't have mounting ears cast onto the top of the center it didn't come out of a coil spring car, so that eliminates A and B-body cars(Malibu/Monte Carlo, Impala), pointing to F- body(Camaro/Firebird '71-81) or possibly X body('72-79 Nova/Omega/Ventura/Apollo)

I'd search F body or X-body forums to see if you can match that stamped code on the axle tube, it will decipher application and gear ratio.

You can also pull the rear cover and check what gears are there and see if it is Posi or Open differential....it probably needs a fluid change anyway, after all these years. ;)
 
#7 ·
The 8.5 wasn't used in the GM trucks until '81 or '82, so it can't be older than that. In '90 they were upgraded from 28 to 30 spline axles. In '98 they were upgraded again to 8.6 and the differential bearings were upgraded to the size of the 12 bolt bearings. Yours is probably a '81 to '87. This is just a guess. There are codes and a casting date on it that will tell you what year it is. Checking the axle splines will get you within a few years.
 
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