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Old 03-17-2020, 06:36 PM
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Jays76bird Jays76bird is offline
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Default GM 10 bolt on 76 Formula gear sets

Hello everyone, asked this question on a Facebook page and one of the posters recommended this forum. Be patient I know little about GM rear ends. So here goes, I have a 76 Formula 350 auto. Came as a roller with a 74 350. Right now it has a series 2- 10 bolt (code PJC) with a 2.41 open rear. I want to go to a 3.08 gear.
A lot of people said I need to go to a series 3 carrier, others have pointed out they make gear sets in 3.08 for the 7.5 series 2 carrier. I don't want to get stuck with a 200 dollar set of gears. There a lot of options out there on Amazon/Summit/Jegs and other places. I know I want to dump the 2.41 gear set. Thanks for any help

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Old 03-17-2020, 06:46 PM
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b-man b-man is offline
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Your ‘76 should have the GM corporate 8.5 10-bolt, your current differential or carrier only accepts 2.41 and 2.56 gear sets.

You’ll have to change the carrier to the 3-series which accepts 2.73 and numerically higher gear sets to use the 3.08 gears you want.

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Old 03-19-2020, 07:46 PM
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Formulabruce Formulabruce is offline
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These are easy to find, Facebook> Secondgengraveyard , send a message to John Delorean ( John Hurd) .
He has a whole rear for like 350 I think.

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Old 04-13-2020, 02:49 PM
Baron Von Zeppelin Baron Von Zeppelin is offline
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You'll probably be happier with 3.23 limited slip , if using a stock'ish 350 or 400 cid.
Unless you just want a low-performance cruiser type - which is okay too.

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Old 04-13-2020, 04:15 PM
'ol Pinion head 'ol Pinion head is offline
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Quote:
Originally Posted by Jays76bird View Post
Hello everyone, asked this question on a Facebook page and one of the posters recommended this forum. Be patient I know little about GM rear ends. So here goes, I have a 76 Formula 350 auto. Came as a roller with a 74 350. Right now it has a series 2- 10 bolt (code PJC) with a 2.41 open rear. I want to go to a 3.08 gear.
A lot of people said I need to go to a series 3 carrier, others have pointed out they make gear sets in 3.08 for the 7.5 series 2 carrier. I don't want to get stuck with a 200 dollar set of gears. There a lot of options out there on Amazon/Summit/Jegs and other places. I know I want to dump the 2.41 gear set. Thanks for any help
First, to the Facebook guys, there are no aftermarket 8.5 thick gears so a 2.73-4.56 8.5 ring gear can bolt onto a high ratio #9 (2.41-2.56) limited slip carrier. Most popularly, the thick ring or thin ring gear deal is a 7.5 GM 10 bolt & 4 series carrier (thin) 3.73 12 bolt car differential deal. Desiring a posi & & 3.08's, your easiest find would be a core OEM 3.08 limited slip rear out of an '80 or 81 Turbo T/A (or Turbo Formula optioned with a posi).

Be advised, nearly ALL of the used #8 & #9 GM Limited slip carriers will need new posi clutch packs & to be properly shimmed up! Through the late 80's, 90's, & early '00's, I routinely pulled this common style 8.5 posi unit, along w select housings & gearsets in the cheapie boneyards, eventually rebuilding the oem posi carriers. Nearly all needed new GM clutch packs & shimmed up properly for performance use. Many seem to skip that final process. Pricewise, the GM clutch packs went from $48 & change, my cost, to chasing them down on parts trackers & paying nearly 3x that much for the last few sets in the 00's. Thankfully, about 12 years ago, an enthusiast in NJ had the square tab style GM S spring posi carrier clutches reproduced, they can be bought for near $200 a set, it's best to search out a seller who offers an asst of thin clutch shims with them. Mention that, though just having a few thin clutch pack shims will not guarantee the ability to shim up the spider gears properly.

Other options are to have the current housing (or a clean swap-out '75-81 housing) professionally assembled with a quality set of gears, all new bearings, races, seals & a new Eaton clutch type posi carrier, or an Eaton TruTrac carrier, or even an Eaton clone type clutch posi carrier. If you desire a new posi carrier, it could be good time to upgrade to new 30 spline axles over stock 28 spline, either Mosers or Dutchman, the latter are best if running hub centric wheels like the cast aluminum snowflake or turbo wheels.

Either which way, on a used rear, carefully remove the c-clip cross shaft bolt, & double ck the factory axles. Very common to see an 8.5 c clip axle bearing (national 5707) track the c-clip axle & create spalling or a groove in the axle. Axle(s) will then need to be replaced. Trying to stay on a budget & only need one 28 spline axle, have out of factory rear disc rears, a partial rack of runout checked nice used factory '79-81 disc axles... when they are gone, they're gone. Factory 8.5 drum brake F-body axles, have maybe a dozen, similar deal, not buying late '70's F-bodys as partscar that much any more.

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