cc89formula

anyone ever put a 3rd gen rear end under a 2nd gen pontiac? ive got the disc break rear end out of my 89 formula that i would like to put in my 78 TA, thanks in advance.

Stephen 87 IROC

It's a weak differential. A second gen is probably an 8.5 diff. I'm not sure the 8.2 was offered. The third gen 10 bolt is 7.5". The 89 will be 7-5/8" and still have the small 26 spline axles. 28 spline started in 1990. If it's a 9 bolt diff, it will be a stronger 7-3/4" diff but parts are hard to find and expensive.

As for if it will fit or not, you'll have to install leaf spring mounts and shock mounts. Not sure on the width. Second gen might be wider.

Rice Killer87

Yea,they are different in size like Stephen said...but what about taking the discs off the 3rd gen rear and adapting them to your 2nd gen rear?

Tru2Chevy

I don't think the 3rd gen rear is too much different in size (width) from a 2nd gen rear. I've seen plenty of second gens with stock third gen wheels on them that look factory fit....

- Justin

cc89formula

stephen: its a 10 bolt and you are right it is a 7.5" and has the weaker 26 spline axles, but this cars never going to see a ton of power and isnt used for racing

Rice: i had thought about just putting the disc brakes on, but that car needs a new rear end, plus its got 2.56s and my 3rd gen rear has 3.27s

as far as size, i know my third gen wheels fit just fine on the 78, and the 2nd gen wheels fit just fine on the 89

Stephen 87 IROC

3.27? Are you sure that isn't a 9 bolt diff? GM didn't put 3.27 gears into a 10 bolt. The Borg Warner 9 bolt has 3.27 gears. It is the stronger diff but as I already mentioned, parts for it are expensive and hard to find.

Third gen diff identification.

10 bolts on rear smooth cover. 7-1/2 or 7-5/8" GM 10 bolt. Many variations through the 10 years of use as GM kept changing things.

9 bolts on rear cover with rubber fill plug. Australian BW 9 bolt. Performance diff option offered from 87-89.

10 bolts on rear cover with a metal fill plug in the cover. Very rare Dana 44 with 8.5" ring gear. Not a factory installation but was once offered through the GMPPC. SLP Firehawks had them installed.

cc89formula

its a 10 bolt...atleast i think so...ill crawl under tomorrow and check, and i remember running the RPO codes for it and getting a 3.27

Kado91Z

it would be a 3.23 if it were a 10 bolt, 3.27 if it were a 9 bolt

cc89formula

its for sure a 10 bolt, and i have the RPO's GW6:axle gear, 3.27 ratio and G80: axle positraction, limited slip. the original rear end could have been replaced, because im atleast the 4th owner of the car. i dont see one that says what rear end came with the car, whether its a 9 or 10 bolt. my RPMs are the same as a friend who has an 88 formula with the 9 bolt and 3.27s, but i guess they wouldnt be much different than the 3.23s....so now im a little more confused...

Stephen 87 IROC

Look for the code on the axle. It will be stamped on the front of the passenger side tube. It's hard to read. At least with that code, you'll know what originally came in that diff. Any previous owner(s) could have changed the internals or the entire diff.

If the rear cover has 10 bolts then it's a 10 bolt. If you really want to know the gear ratio, you'll need to pull the cover off and count the teeth. Just because an RPO or axle code says once thing, that doesn't mean that's what's in the diff. Parts can be easily swapped.