Just out of curiosity, were any 85 irocs known to have the borg Warner 9 bolt rear end? Because mine does although not sure if its original. When I went to replace the rear brake lines and diff gasket, they only show at part stores for 85 firebirds and 86 for the camaro so it got me thinking. My car is an original lb9 auto, with G80, GU6, J65 RPO codes.

 

I read somewhere they were sometimes installed as warranty replacements on cars that had problems with their 10 bolts. Or it could have been swapped from another car at some point.

 

Thats a possibility it was warrenty work cause if the original owner swapped it i dont get what the point be would if the car already had a disc rear and the Gu6 performance gear from the factory.

 

To my knowledge, none came that way. The 9-bolt was introduced in 85 in Firebirds, Camaros didn't start getting it until 87.

As said, there's been near 4 DECADES between then and now; plenty of opportunity for some swappage, either by stealerships or by POs, especially given that the usual 10-bolt in such a car and a typical 9-bolt will directly interchange, if they both have J65 (rear disc brakes). "Codes" and all such as that are a total waste of time for telling what you have NOW, they are only good for telling how the car came.

Yours seems to fit into that latter category. GU6 is a 10-bolt "code" if memory serves, for 3.42 ratio I think but could easily be wrong. 9-bolt "codes" were different, not least because their ratios weren't exactly the same. The "code" you have clearly states that it came originally with a 10-bolt. The "code" doesn't change when Skillet goes to the junkyard and grabs ... whatever. Or when the stealership does the same thing with whatever GM tells them to do.

Who knows what you've got now. Best bet is, since you OBVIOUSLY haven't done the most basic and elementary MAINTENANCE that so often gets deferred, is to catch up on however many million miles of "services" that haven't been done, and while you're changing the rear end grease AS YOU SHOULD ALREADY HAVE DONE AS SOON AS YOU GOT THE CAR, make a point to LOOK AT the gears and see what you've got. I'm guessing it'll be AHELLUVALOT less than a 3.42 now. Which sux, butt, it is whatever it is, independent of any "codes" or anything else related to "original".