Rear Axle ID

My rear axle was supposed to be a 3.73 posi GM 8 1/2" but, I fear it may actually be a GM 7 1/2". I measured the ratio at 3.55 with non-posi. So, what rear is this? And, if I have to go on the hunt for a proper 8 1/2" 10 bolt which are the ones that will fit properly like the original in my '71 GS?

 

That is a Pontiac 10 bolt 8.2 rear. If you like the gears it is very easy to just make it posi. 3.55 is a good all around gear set. Pontiac has the strongest 8.2 as the caps are very stout and casting is well designed diagonal ribbing. Jim J D

---------- Post added at 11:33 AM ---------- Previous post was at 11:29 AM ----------

I guess you would have a 1971 or 1972 Buick Skylark then?

If you are looking for a 8.5 you will only find them in that 1971 1972 Skylark and Cutlass. I have seen them in some Pontiacs in the same 2 years also. Lemans ...So that is it. Do remember that any A-body could have one under it so in the junk yard do not pass over any 68-72 A-body CHECK them all you never know.

 

Thanks, Jim. That's very encouraging as I was afraid it was a measly 7.5". And, I'm happy with the 3.55 ratio. So, how do I tell the difference between the GM corporate 7.5 and this 8.2? The "tabs" at the bottom corners sure looked like the ones the 7.5 is supposed to have.
Several questions remain:

1) So, all I have to do is find a posi carrier for the 3.55 gears? Do you have one?
2) Is this rear end housing the same outside (drum to drum) dimension as the '71 Buick GS 10 bolt that came in it originally? I want to be sure as it will affect the backspacing on the wide repro wheels I'm getting.

As always, thanks for your help. :TU:

 

Hey Jim how can U tell the Pontiac 8.2 from the Buick 8.2? I thought they were the same?

 

yes I can - Just post the pictures and will get right on it. Jim

I have just seen so many GM rears it is a WEIRD knack...LOL. Jim

---------- Post added at 04:04 PM ---------- Previous post was at 04:01 PM ----------

The castings are different. If you check the site you can see that. They actual casting is thicker and more rounded off in the 8.2 Buick the Pontiac has shaper edges where it meets the tube... Plus with the help if where casting numbers are located in the picture and other details (if I was unsure ) I would ask you for. Jim

---------- Post added at 04:10 PM ---------- Previous post was at 04:04 PM ----------

7.5 compared to 8.2 rear cover is EXTREMELY different. You just compare the covers and your done. (pictured below)

1) I have a reconditioned Pontiac Super Duty 4 pinion posi for 325.00 (we will need to know what RING GEAR bolt size you have in that rear. So you will need to remove the cover and see if the ring gear bolt head is 11/16 or 3/4" We have both units so that is not a problem
We can also get you a NEW Nitro Eaton posi for 399.00 Which is the best for street strip car. Will bolt right up and is fully rebuildable down the road too. Not to mention very strong design.
2) 1967-1972 A-body rears are ALL THE SAME WIDTH 55 1/4" BACKING PLATE TO BACKING PLATE (HOOKED MEASUREMENT) so that will solve the rim wheel trouble some thought. Jim

 

So, how do I tell if my Pontiac 8.2 10 bolt is a pre 1967 or a 1967-1972 so, i'm sure it is the 55 1/4 width? I understand that the early 8.2 rear ends are narrower.

 

Measuring is the first step. and probably the most important for fitment. 55.25 is the 67-72. I have seen 1966 also wide with FLAT perches...I did not think that existed either BUT they do. Factory welds etc. Very late 66 and or early 67 flat perch but is 55.25 wide. Next the casting number will help found in the lower right rear webbing (your picture) Then the ring gear will also have a date code on it. Ring gear bolts for the 1970-1972 pontiac will be Left hand thread. ALL in all it is NOT real easy if you do not look at them all day long. Jim

I can not tell you I still am learning and just when you think you have it all down on paper something comes along that should NOT BE but is. ???

This is why I prefer pictures to help ID the rear. The early Buick and olds rears have different seals and yokes than the Pontiac rears.

I can tell by seeing the yoke and the casting of the rear center section. Jim

 

So, how much narrower are the early rears? And, when you say 55.25 "hooked measurement", what do you mean?

 

HOOK- Drum backing plate to dtrum backing plate. UIt is hard to measure flange to flange.

54.25 flange to flange (1" narrower.)

Jim

 

Thanks, Jim. We'll measure it so I can decide whether to put your Eaton Posi unit in it or go looking for a 71-72 Buick 10 bolt.