Hello,

I have recently disassembled 2 GM rear ends, one from a 1971 Cutlass Supreme with 2.56 axle ratio (41 tooth ring gear) and also a Buick Special with 2.73 gears (also a 41 tooth ring gear. To my surprise, both ring gears measures 8.2 inches, not the 8.5 that I was expecting. I pulled the Cutlass axle myself from a car we parted out. The Buick I purchased by itself. Did GM by chance use an 8,2 ring gear in any of its corporate axles with numerically low ratios? My intent was to use one of these axles in my 1966 442 with a posi unit and 3.42 gears with hopefully the stronger 8.5 inch ring etc. Since I am basically starting from scratch, my main interest is in the housing and axles. Any help would be very much appreciated. Thank you, Larry.

 

Post up pictures of the axle housings. My original 1971 8.5" corporate 10 bolt 2.56 rear has a block O on either side of the center section where the axle tubes attach.

 

Hi,

I will attempt to attach pictures of the axle housing with this note. Here is some other information:

Center Section Info: Has numbers 1235197, 13, and letters CFD and either a backwards C or a sideways U.

Axle Tube info: Has R2 B139 1.

Again, I am hoping this is a "corporate" axle housing but am confused as to why the ring gear was only 8.2 inches. It was removed from a 1971 Cutlass Supreme with 350 4V motor. Ratio was 2.56 (15 to 41).

Thanks in advance for any help you can offer. I want to run a 8.5 posi unit but want to make sure the housing will accept it. THanks. Larry

 

I looked at this with Larry and was surprised to see the ring gear only measuring 8.2 inches across. Could it be that the highway gears used a smaller ring gear in the same housing that the more performance ratios used 8.5 ring?

John

 

From the shape of the cover, that appears to be an 8.2 housing, same as the ones in my 66's.

 

Yes GM made an 8.2" and an 8.5" rear. The 8.2s generally went into grocery getter 6 cylinders and 350s. The 8.5s went into the higher performance cars.
An 8.5 ring will not fit on a 8.2 carrier nor its housing.

 

Just to clarify, the 66's came stock with the 8.2 carrier. I have never broken a Ring Gear, but once twisted off an axle, and I did toast a set of spyders.......... but in all honesty that was after the equivalent of probably a thousand passes (I used to beat the living daylights out of that car)

 

I am really getting confused here. I did a bit more research and was told that the 8.5 Corporate axle assembly used left hand ring bolts measuring .4375 while the 8.2 used right hand ring bolts measuring .375 inches. My ring bolts are left hand threads and .4375 inch diameter. Was also told the 8.5 pinion measures 1.625 diameter while the 8.2 measures 1.435. If this measurement is taken just above the big bearing, then mine measures 1.625 there and tapers down towards the front. I was wrong in my initial posting with the number of teeth on the pinion, it has 16, not 15. The pinion measures approx 8 3/4 inches long and has #1234770 and the letter H. The center section is approximately 12 inches from front to back, all gears removed and the snout on the floor, measuring from floor to where the cover bolts to. So, back to the original question; did GM install any 8.2 ring gears in 2.56 or 2.73 ratio intermediates in 1971? As good as I can tell, I seem to have a Corporate axle housing with an 8.2 ring gear. Any thoughts would be greatly appreciated. Thank you. Larry.

 

This is not correct info... Sorry. Jim
8.2 was the only rear till 1967 in the A-body for B.O.P
Then late 67 Olds made a 12 botl 'O' with 10 bolt guts.
Buick made a 8.25 Big 1.875 Pinion inner bearing rear with a large carrier bearing on the driver side 68-70
Pontiac kept using the 10 bolt 8.2 P rear till 1972 and used the 12 bolt Chevy in the special GTOs
BOP used the 12 bolt chevy in 68-70 for some Canadian cars for a base rear too.
1971 1972 were the only years for the corp 8.5 rear in the A-body GM
1970 there was a GM strike and it was a messy time for parts being interchanged from a-body chassis's all over the place Mone carlos El Caminos etc had Tempest 8.2 rear in the cars ...What a mess...

Jim

 

This rear is a 8.5 corporate rear HOUSING. I do not know what gears were in the housing will need to see them and run the numbers
IE ford 9" rear had gears that only measure 8.75 some were just not the diameter - depends on the year and the ratio etc.
NOTHING is AN ABSOLUTE IN THE WORLD OF REARS.

THE HOUSING IS A CORP 8.5 71 72 rear. Period.

 

R2 is a Oldsmobile 10 bolt 8.5 2.56 rear

The yoke will have 30 splines on the pinion

axles will be 45 degree cut 28 spline bolt in axles

ring gear bolts will be 7 /16" x 20 left hand thread

The way the brake hose has a boss and drilled and tapped spot on the top of the housing is 8.5 corp ONLY... 8.2 chevy or pontiac or buick all were a bracket that attached to the rear cover bolt.

Finally your casting number is 8.5 corporate... were done here.
10 bolt 8.5 71 72 Oldsmobile rear end with 2.56 from the factory open non posi rear.

 

Jim,

Thank you so much for the clarification. It is very much appreciated. Larry.

 

some day, someone will put all this sheet on a spread sheet! By year, By model, By application...! And be done with it!

 

Unfortunately it will not be me... When you have to make a living do it ...it does not quite make sense.

I know that seems snooty But if you have certain knowledge in the job you do for a lawyer or auto mechanic or anything you might do for a living it keeps your job stable and you deserve the advantage as you worked to get that advantage in your job.

Jim

I never have a problem helping ID and helping with setting up rears...

 

Wow, the timing of this thread is pretty good. I am trying to ID this rear under 1971 Cutlass.

It seems to have the "classic" 8.2 cover, yet the protrusions at 5 and 7 o'clock that I've been told indicate a corporate 8.5. I know things got mixed up a lot during 71... For example, USA Standard Gear actually shows the two options in the other image.

I realize it is just a picture, but can anyone give me more information about it? I can get the numbers and even pull the cover during the weekend if that will help.

Thanks!

 

From what I have learned, pull the cover and check out the ring bolts. If they have a cast "L" on them, meaning left hand thread, it is most likely a 8.5 corporate even though the ring gear may be smaller.

 

Read the information that has already been posted and the answer will be evident.

 

http://www.oldcarmanualproject.com/m...Ill/Group5.pdf

 

No worries Jim just frustrated with this subject. Liken it to the Offset Supreme engine for header purchase...oy! I didn't read the OP thoroughly hence the jack rabbit answer. Not good info. I am not an expert but have had a wrench in my hand since the 70s, professionally at times. I too enjoy helping out.
I just found a good link... see the previous email above. I never bitch unless I have something positive to add towards the solution...
As an engineer it would be nice to have an expert in 60-70s Olds rear ends, corporate and non, broken down into its constituent parts on a spread sheet as that's they way us engineers have to see it...ring size, bolt size & thread direction, spline count, pinion nut size, seal and bearing numbers etc... etc...all in one spot. Im versed with the 8.5" 10 bolt ring geared azz end found under late (I believe) 67- to early (I believe) 70 442s AKA... Olds "O" type 8.5" with the axles bolted in at the brake plates with the pressed on outer sealed bearing.
The link I provided has some useful info check it out.

 

I have a spreadsheet with some info on the O-Type (early, late, and HD) and the corporate 8.5" 10 bolt. I have at least one of each of these rears and I logged the part numbers off the bearings, seals, carriers, etc along with some descriptive info. It's not a huge amount of info but it has some of what you were mentioning.

 

Hello
your axle have C clips? also the yoke is 27 or 30 splines?
Thanks,
Juan

 

Hello
I have the same axle,this have C clips or not,its different for chevy?
Thanks,
Juan

 

THE HOUSING IS A CORP 8.5 71 72 rear. Period.

This is a very OLD post ...You really should post pictures of your rear for ID if you need help.

Then it will not get all mixed up like this post is.
Jim
JD