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Discussion Starter · #1 ·
ok, heres my story. ive got a 76 gmc sprint sp with a 10 bolt open style rear end. the gears are real crappy 2.73 or higher. my plan is to use the rear end out of another sprint i have. its got a 10 bolt open style with 2.08 gears. (41 on the ring, 15 on the pinion) im assuming this rear end is the 8.5 style. im going with a 3.73 richmond gear, but need dome help about a posi. everything ive seen for this rear end says the posi unit is for 2.73 and higher gears. obviously the 3.73 is lower, so what can i do? does anyone know of a posi unit that would fit? thanks
 

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Go here for how to tell an 8.5" rear end from a 7.5" http://tech.oldsgmail.com/axle_ident.html

If you're changing gears & getting a posi at the same time, just be sure the posi carrier matches the new gears. If you're going with 3.73 gears, get a 3-series posi. If you want a used GM unit for an 8.5", be sure it has the number 410408 on it. This is the 3-series GM posi carrier, which (on an 8.5) is used with 2.73-up gears. If it says 410409, it's the 2-series, which is for 2.56-down gears. 3.73:1 is a higher ratio than 2.73:1, and that's what the manufacturers mean when they say 2.73-up and 2.56-down.

To get the ratio of those gears, divide 41 by 15, which comes out 2.73.
 

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You can also buy the 3.73 gears for a 2 series carrier.

Many of the different R&P manufacturers make the high ratio gear sets 3.42/3.55/3.73/4.10/etc. for the 2 series carrier.

As far as which posi/limited-slip/etc. to get, go for the best deal you can get by shopping around, some places will definitely have a better deal than others as they buy in larger bulk numbers.

If you are swapping the gears yourself, get the whole package deal, meaning-the ring & pinion gears/differential carrier/ and complete gear swap setup kit. These are always the best deals.
 

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Discussion Starter · #4 ·
thanks for the info, it helps explain alot. i forgot to ask about the number of axle splines though. ive noticed that all the carrier units require either 28 or 30 spline axles. is there anyway i can find this out without pulling the axles?
 

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The 30-spline units were not used by GM until 1988 or 1989, & then only in trucks & a few full-sized cars. If the rear end has not been modified, it should have 28-spline axles. Wouldn't hurt to count them anyway, but count twice because your eyes can play tricks on you.
 
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