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Old 09-15-2004, 04:53 PM
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1975 8.5 rear end

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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Old 09-15-2004, 11:50 PM
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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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Old 09-16-2004, 05:08 AM
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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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Old 09-17-2004, 05:53 AM
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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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Old 09-17-2004, 06:32 AM
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The earlier axle were all 28 spline.
The only positive way to tell is open it up.
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Old 09-17-2004, 08:49 PM
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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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Old 09-18-2004, 05:32 AM
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The easiest way I have found to count splines or gear teeth is to use soapstone, and mark every fifth spline or gear.

I'm getting older and need things like this to prepare me for the future.
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