Just new to the site Have 85 firebird 350 crate motor with 5 speed Want to change rear end? Was wondering will a 10 bolt out of a 70-81 trans am work? Going to get frame connectors put in and going to use southside bars and new panhard bar.

Not a direct bolt in if that is what your looking for. the second gen rear ends don't have the places forthe LCAs and Torque arm. I would recomend if you are going to to that kind of fab work on it just do it to a 12 bolt or a 9inch ford not another 10 bolt. However a 10 bolt out of a Forth gen camaro or firebird would be a direct bolt in.

First and second gen f-bodies used leaf springs. Third and fourth gens use a torque arm suspension. Although all the mount brackets for shocks, springs etc can easily be cut off the third gen diff and welded onto another diff, there's no easy way to attach the torque arm onto any other diff other than a 9" by using a special bracket available from Currie. A fourth gen diff isn't much stronger than a third gen diff and it's slightly wider. Your 85 will have 26 spline axles and the 1990+ f-body diffs have 28 spline axles. Other than that, they still have a weak 7-1/2 (7-5/8) ring gear.

Before trying to swap over some other diff, make sure it's close enough in width to a third gen diff.

If you search through the forums, you'll find the South Side bars are not recommended and most third gen owners find them as junk.

BMR actually makes a bracket for 12 bolts it's not designed to work with a third gen. i believe it's used for a torque arm conversion for first gens. But they do make a way to put stronger rear ends in to third gens, if you have fab skills you could make the same bracket if you want to put that kind of time and effort into it.

Info in the following FAQ may prove useful:

http://www.thirdgen.org/techboard/tr...-10-bolts.html (FAQ About 10-Bolts)

JamesC

How fast can you go with a 10 bolt rearend?

Which 10 bolt? There are at least 3 different 10 bolt rear ends. The only 3rd gen 10 bolt is the 7.5, and it isn't very strong.

I was somewhere in the mid to low 10's using a beefed up 10 bolt before finally switching to a 9". I installed a transbrake and knew the 10 bolt wouldn't take more than one transbrake hit so I changed the diff at the same time.