I'm looking to upgrade the rear in my G.P. from a 2.56 open to a posi in the 3.08-3.23 range. It doesn't have the corporate 8 1/2" 10 bolt. According to the 75 Pontiac service manual it has an 8 7/8" ring gear and a 10 bolt cover with a drain plug. I think this is some kind of 12 bolt carrier but unsure. Would like to know if anyone has any knowledge of this rear such as parts availibility or would I be better off trying to find an A body corporate 10 bolt?  

fireprix:
I'm looking to upgrade the rear in my G.P. from a 2.56 open to a posi in the 3.08-3.23 range. It doesn't have the corporate 8 1/2" 10 bolt. According to the 75 Pontiac service manual it has an 8 7/8" ring gear and a 10 bolt cover with a drain plug. I think this is some kind of 12 bolt carrier but unsure. Would like to know if anyone has any knowledge of this rear such as parts availibility or would I be better off trying to find an A body corporate 10 bolt?

I have the same car and the same problum.  Can any one anser this qustion  

I have been doing ALOT of reasearch on this. G.M. used different versions of this rear. Unfortunately the aftermarket doesn't make gears or carriers for this type of 12 bolt that I have been able to find. The G.P. version is considered a very stout unit due to it's 31 spline axles. C Clips are the only drawback to it. This rear is commonly found in stationwagons, full size and some mid sized G.M. cars. It is a 12 bolt carrier with an 8 7/8" ring gear. Carriers can be rebuilt but I've been told spider gears are no longer avaliable. G.M. only offered ratios upto 3.42 for these but used 2 and 3 series carriers. Most of these rears built from 71 to 76 are the same in Buicks, Oldsmobiles and Pontiacs but Chevys may be different. Anyone who has these parts laying around or knows of a car that has a 3 series posi in it please post. Due to the number of reads and another Pontiac fan posting the same problem any information or leeds would be greatly appreciated.