How can I be sure I bought a posi diff.?

Alright so I bought an 8.5 10bolt 28spline posi for my 74 Nova which is supposed to be out of a Firebird.It came with what I was told were 355 gears but now that I have it I counted the teeth and believe it is a 308!My question is how do I tell for sure the diff. is even a posi as it hasnt been installed yet and can you even get 355 gears for my diff. or only 342 or 373?Please any info would help.THanx again guys:shakehead

The quickest way to tell is probably to throw the axlesin it and spin to see if both axlesturn the same direction. As far as gears I'm notsure, I think for the right priceyou canmake anythign happen. Thedre are a lotofunitsavailableon Ebayfrom shops that will assist you in determining what will/ will not fit in your housing.

If you've been "in there" to count the teeth, you can easily see what type of carrier you have. A "posi" has clutch packs betweeen the side gears ("spider" gears). An "open" axle has only gears.

3.42s are the "popular" gearset for the 8.5. They're "close enough" if you're after 3.55s. I don't know that anyone offers 3.55s for that axle.

Jim