How do you measure if 8.5", 8.2" or 7.5" 10-bolt rearend - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-29-2004, 12:12 AM
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Talking How do you measure if 8.5", 8.2" or 7.5" 10-bolt rearend

I'm sure this may seem easy, but how do I identify the size of the factory 10-bolt, 2.73 open differential on my '79 Formula Firebird?

Is it the max diameter of the ring gear, the carrier or what? Have a nice fresh 400 engine that I can't get to hookup at the track. I have access to several mid 70's Firebird 10-bolt posi axles, but not sure how/what to measure to make sure I get the strongest one...

Also what about just adding a Powertrax differential in my existing 2.73:1 10-bolt? Thoughts?

Thanks!
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Old 04-29-2004, 07:06 AM
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You have the 8.5 inch ring gear which should also have 28-spline axleshafts. Pretty desireable considering the others GM had in that timeframe.

This axle was found in 1973 through 1981 Firebird and Trans-Am, among other cars.

The 1982 through 1984 would have the 7.5 inch as stock, and 85-87 would come with 7.5 inch if it has the 10 bolt cover, or 7.75 inch (Austrailian) axle which had 9 cover bolts (weak axle, and harder to find upgrade parts for).

Adding the PowerTrax is a do-it-yourself type of install not requiring special tools, skills, or setting up the wear pattern on the ring and pinion gears. They are dependable units and used quite frequently in off roading situations.

They are a little noisy though, the only downfall I have heard of yet.
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Old 04-29-2004, 10:26 AM
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I can say it's an 8.5 unless someone switched it. The easiest way to tell what you have is to get under the car and look at it. See which on of these two it looks like. http://tech.oldsgmail.com/axle_ident.html


Mike
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Old 04-29-2004, 06:17 PM
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Thanks Ghetto Jet I will add that link to my axle info sticky post at my next update.
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Old 05-05-2004, 10:49 PM
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Thanks everyone for your help!

Thanks everyone!

I'm going to retain my original 8.5", 28 spline axle (w/triangles) and go with the Powertrax posi differential.

They have a newer one with synchro's that's not supposed to make the ratcheting sound common to the off road units.

Will advise when I get it in and take it out for a few runs!

Sincerely,

KC
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