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67MercCat 11-11-2012 12:20 AM

12 Bolt posi unit
hey guys, do you know what car or truck I can keep an eye out for in the junkyard for a 12 bolt posi unit? This is going into a 1966 Chevy Caprice. Also do you know what the spline count for a factory 12 bolt rear end.

Thanks fellas 11-11-2012 07:45 AM

65-74 Chevrolet mid SS and large body/big engine cars. 68-72 NovaSS 67-70 Camaro large engine, Some performance 68-74 Pontiac GTO, Olds 442, Buick GS. but rare.

Truck/van 12-bolt is not the same.

Doubt if you find one. I know the yards here have no early cars because of scrap prices.

MouseFink 11-11-2012 07:58 AM

IMO....The 1965-1972 Caprice wagon, Impala, Biscayne, and Bel Air 12-bolt rear ends will interchange with a '66 Caprice and they use 30 spline axles. The 1965-1972 Pontiac Bonneville, Olds Delta 88, Buick Electra 225 and Cadillac Brougham 12-bolt rear ends and they use 31 spline axles. The 1973-up GM 12-bolt rear ends will not interchange with any 1965-1972 GM vehicles in any way.

That is part of what I found out when I was sorting out a Chevy 12-bolt Positraction for my '66 GTO. I finally gave up and installed a 1965-1967 4-pinion, 3.90:1 Saf-T-Trac for a Pontiac Firebird, GTO,Tempest, LeMans differential 10-bolt rear end.

big gear head 11-11-2012 08:07 AM

You will not find one in a salvage yard. The Chevy 12 bolt rear ends are very hard to find now, and I know that there are people who are being paid to search the scrap metal piles for any Chevy car 12 bolt rear ends that they can find. There is a guy here in KY that has several people in many different states that he is paying to search for them.

The Chevy car 12 bolt was only produced from '65 to '72. It was used in the full size Chevy from '65 to '70, the Camaro from '67 to '70, the Nova from '65 to '70, the Chevelle and El Camino from '65 to '72 and the Monte Carlo from '70 to '72. There were a few GTOs that got the Chevy 12 bolt, but not many.

The truck 12 bolt is similar, but the ring gear flange on the differential is offset different than the car 12 bolt. The Eaton Posi that was used in the truck 12 bolt was weaker than the car Eaton Posi.

Why not just get a new Eaton Posi and have it done? There is also the Yukon Dura Grip, which is an almost exact copy of the Eaton Pois for a little less money. If you find a used one you will probably have to rebuild it anyway.

fy54957 01-19-2013 11:57 PM

What would be a good prices for a 12 bolt rear end .

big gear head 01-20-2013 08:04 AM

Depends on what is in it, what it came out of and what the condition is. A rebuildable core for a '69 Camaro would go for around $500 if everything in it needed to be replaced. A completely rebuilt Camaro 12 bolt would go for around $1700 if it had a good gear ratio and Eaton Posi. A COPO Camaro or Chelvelle rear end would go for around $12000. A '66 Impala core would sell for around $250.

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