I have a v8 firebird 1989 automatic, and i want to change the gears and put a possi in, now i just have no idea if the axle is a 26 spline or 28 spline, or what do i need to know so i can buy the possi and the gears?

 

Gears, the splines don't matter.

Posi, you need to buy the right series to go with your new gears.

As far as splines, there's no way to know what you have. Even if it was ABSOLUTELY POSITIVELY CERTAIN that it was the original rear, you can't be sure, because late 89 was when the changeover occurred. Something around 80% of 89s, or maybe more, are 26 spline. Not much of a guarantee one way or the other.

So, you could decide you want 3 series gears, let's say, like 3.42 or whatever (3.23 or higher); get the posi for that, in the 28-spline version; and just buy 28-spline axles to go with it. http://store.summitracing.com/partde...SAG%2DEV10%2D4

 

That's providing you have a 10 bolt and not a 9 bolt diff.

 

ok so if the splines dont matter, i found 2 kits on ebay, they are both used and come with the posi locker and gears, same seller has 1 26 splines and 1 28 splines, so they will both fit cuz they come with there gears already?

these are the item numbers of the posi lockers:
120230785035
120230785797

and are posi lockers good, i heard the limited slip is better???

the auction is almost over so i need some help now please....


thanx a lot

 

No... gears are not connected with axle splines. The same gears will fit a carrier with either axle spline count. You need to find out what axles you have, and buy a carrier that either has the same splines (and the same gear "series", which is a whole other matter, not related to splines); or, buy axles to match the carrier you buy.

No.... "posi" is merely slang for "Positraction", which has been a GM trade name for "limited slip" at times over the years; just as Sure-Grip, Traction-Lock, Quadra-Trac, etc. have been similar trade names used by other companies for the same thing (limited slip). IOW, "posi" is just an alternate spelling of the words "limited slip". A "locker" is something else again.

All of course, assuming that YOU have a 10-bolt, and that whatever carrier you buy, is also for the same model of 10-bolt.

 

Splines don't matter for the gears. Splines do matter for the posi.

Your 89 will probably have 26 spline axles but the only way to know for sure is to pull the axles and count them because it was a changeover year. If you upgrade to a 28 spline posi, you'll need new axles if yours are only 26 spline.

GM produced 3 different 10 bolts. 8.2", 8.5" and the small, weak, 7.5". Third gens that have a 10 bolt diff are all 7.5" versions.