ok heres what i know.1987 z28 305 carberated.what im pretty sure of,2.73 non posi rear.first im new at all this i used to be in the import scene so be gentle and use small words. lol.how do i tell which bolt rear i have?is it worth just buying a stock posi unit and putting that in the rear?any help you can give me is appreciated.

 

Count the number of bolts that hold the cover on the rear of the diff

Easy huh...

What you really want to do is check for the axle code. This should be located on the passenger side front of the axle. It's probably covered in crud so you may need to wipe it down a bit.

If you can find it post it here and someone should be able to tell you what axle and gears you have.

As far as the stock posi goes, I really can't answer. I just bought a 9-bolt axle complete with the gears I want and a locker, and disk brakes to boot.

Another thing to check for is any tags bolted on the cover. The diff I just bought has both the post tag, and a tag with the p/n and gear ratio.

 

Forgive the embarrassingly obvious, but... you count them. 10 bolts holding the cover on = 10-bolt; 9 bolts holding the cover on = 9-bolt.

Yes, a stock posi unit (while certainly not the best posi there is) is alot better than no posi at all. The things that you need to watch out for are that it has to be 2-series (for 3.08 and down gears), and 88-back with 26 spline axles.