I am having trouble determining which differential I have. We bought the Iroc Z28 camaro from a friend of mine. He said he thought it came out of an 89 firebird. It is a posi-trac, that much I have verified. It is also a 9-bolt pan and has rear disc brakes. We had to replace the axle bearings about a year ago, but our local parts dealer did a lot of cross-referencing with the bearing and the bearing race to get a set that would work. I am wanting to get a pan gasket and know what differential I have in case I need to get additional parts. The tag is not on it and I am not having any luck finding casing #'s. If anyone has any suggestions, I'd greatly appreciate it. Thanks, Pat Rogers

If the cover has 9 bolts then you have a Borg Warner 9 bolt diff only available from 1987 to 1989. The 89's will have the large rotors with aluminum calipers.

I don't even bother with diff cover gaskets. Run a bead of silicone all the way around it and bolt the cover on.

Thank you for the reply! I really appreciate the info. If I may, I do have another question. I was also told that Zoom 380 gears were put in the rear-end. It seems like I read somewhere that there was a fairly easy way to get the approximate gear ratio by turning the rear wheels and counting the revolutions of the driveshaft and dividing by some number. Is this correct? Also, did all of these use the same size ring gear or is that dependent upon what has been put in already?

Thanks so much for your time.

Rough guess is to rotate the yoke and count the axle rotation but it's better to just pull the rear cover and count the teeth.

There isn't very many parts available for the 9 bolt. Factory gear ratios were 2.77, 3.27 and 3.45. An optional 3.70 was available from the GMPPC years ago. You could order other gear ratios from Austrailia where the diffs are still in use.

The 9 bolt uses a 7-3/4" ring gear and shares no parts with the 10 bolt that's also found under third gens. It's like asking if parts from a Ford 9" will fit a GM 12 bolt.

Thank you again for the wealth of info! I truly appreciate it all. This is a really great forum and I have a lot of reading to do!!

Thanks again.