ddusty1

72 Olds CS with a 455 and a TH 400. Been running OPEN rear with 2.71 Expressway gears. I have really been wanting a 12 bolt posi setup and I have been wanting to go 3.08 to 3.23. I run an AC at times and I figure that would be a great leap from the 2.71 and with the 455 for street use, the gears would be plenty.

OK, I have an offer from a very reputable driveling specialist locally to keep my 10 bolt and throw in an Auburn with new gears etc... Complete rebuild(bearings etc...)and out the door for 1200.00

What would you do? On one hand, I just put new brakes etc... On the other hand, I wanted a 12 bolt. I dont race her so the 12 bolt is more for show than anything.

Thanks guys.
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Redog

Well good luck a 12 bolt, since they have found a value in the racing world, they are very hard to come by.

I would go with the Auburn. The price sounds a bit high, but if you want to have a pretty soild set up. Sounds like it would be good for racing, but if you're not racing the car, or it's seeing rare track use, maybe the currect rear is fine

3.08 would be the cheapest way to go, since you can use the same carrier. 3.23 and up use a 3 series carrier

Rallye469

Seems to me like your stuck between 2 problems.
1,200 bucks to convert your 10 bolt seems a bit high.
I converted mine for $850 with parts and labor.

But you'll probably spend close to close to 2 grand buying an
aftermarket 12 bolt from a vendor.

Best bet, comb the local want ads for one, see if you can't find a deal.

Run to Rund

Before you choose something, call Doug at Quick Performance 515-232-0126 (www.quickperformance.com). They mostly do 9" Ford rears and have bolt-in units for your Olds, various prices depending on what equipment you want. At least you would have the strongest starting point for any future upgrades if you try drag radials etc. in the future. I wouldn't put $1200 into a 10 bolt. There are enough of them left in mild ratios in boneyards, and the only really stout and positive positraction is the Detroit Locker, which you can't get for GM rears but can for the Ford. Don't get nervous, Olds bought their HD 3 speeds from Ford so Ford is a legitimate drivetrain vendor for your Olds.

gearheads78

Don't you have an 8.5" 10 bolt it your car? They are pretty close to 12bolt strength and parts are cheap. I would use an Eaton instead and go to at least 3.23 gear ratio. Good used gears are everywhere 75-100.00 out of late model Chevy trucks and SUV's
Get with Brian Trick for a great deal on a Eaton Posi. ROP is down right now so I can't get direct contact info. If you do a search there when its up it should be easy to find.

monzaz

Gearheads78 is correct

You 8.5 will do the job on the street and at the track and it is what the car came with. 10 bolt 8.5 rears are still used today under 1/2 ton trucks. Do you think GM would keep a rear around for 3 decades if it was a bad design or weak? 8.5 was created as the best of everything out there in the muscle car days. all the 1970 -1996 cars were optioned with this rear.

I build the 8.5 10 bolt 71 72 rear for the buick guys all the time. 455 buick motors pound them and they hold the drag only cars we put 30 spline custom axles in and 30 spline eaton unit and they love them. They are also c-clip eliminated rears because the axle bolt in unlike a chevy 10 or 12 bolt.

10 bolt 8.5 rear use all the same pinion bearings and have the same pinion shaft size as a 12 bolt. carrier bearings are standard 10 bolt and the ringf gear is only slightly smaller in over all diameter...but in drag race application it does not mean anything ...larger ring gear diameter was designed to hold down heat like in circle track and heavy truck application. 12 bolt vs 10 bolt

the 12 bolt have smaller 3/8 bolts compared to 2 less larger 7/16 bolts...EVEN ford 9" rear is a 10 bolt rear it only has 10 bolts holding the ring gear in place.

Think twice before ditching the 8.5. If I were closer i would buy it from you to have as a core. 1200.00 is a little steep if you are supplying the housing but is the shop doin the work in the car or the rear out of the car?

carrier brakes remember that 8.5 have a different carrier break that most GM diffs.

2.41 2.56 is considered 2 series
2.73 2.93 3.08 3.23 3.42 3.73 3.90 4.10 and on are all 3 series gear sets.

call if you want prices on parts
Eaton type posi 389.00
gear from 139.00
rebuild kit 89.00 (does not include bolt in axle bearings) 65.00 extra for the axle bearings and seals

that is 617.00 in parts alone. If you think you or some one can do the install for 300.00 they you have just saved 300.00 . Jim

ramnation1

Saw this link... should help a little the other suggestions are right on the mark... befriend your locals and see if they can head you in the right direction -- i.e. turn around, quality, price, and longevity--
http://www.chevyhiperformance.com/te...all/index.html

joe_padavano

I would also say that unless you plan to run slicks, the ten bolt will be fine. The brake backing plates are the same on the 10 bolt and 12 bolt so if you did want to switch, everything swaps over. In fact, I've done this swap without even needing to bleed the brakes. Just pull the axle shafts, unbolt the backing plates, bend the clips holding the brake lines to the axle housing, unbolt the brake junction block on top of the center section, and swap the housings. Reverse the process and you're done.