GM 12 bolt rearend questions concerns
 

i currently picked up a GM 12 bolt rearend for my chevelle

my concern is these articles that say they replaced the strait line rearend(12 bolt) for a 9 inch. can someone please tell me why they are saying this rearend isnt good for a corner carving GM A Body. thank you

What articles???

There are a lot of guys running 12 bolts on the auto-x and road. It's only 1/4 smaller then the 9 inch. You will be fine with the 12 bolt but as David said what articles?

There is a good run down on 12s and 9s from jason at the GMR on my thread here

https://lateral-g.net/forums/show...t=37889&page=2

If you already have a 12bolt, I certainly wouldnt worry about it. Not a great deal in it really.

The only real advantage of the 9" that cant easily be replicated into a 12bolt is the easily interchangeable third member allowing easy track side ratio changes.

Amit

the 9" axles are retained by pressed on bearings instead of c-clips eliminating side to side axle play. thus the 9" doesn't suffer from pad knock back and has positive axle retention in the event of breakage.

if you are going to buy aftermarket axles, moser makes a kit that converts the 12 bolt over to 9" ford style axle retention.

The 9" definitely has pad knock back issues, which is why Mark Stielow and companies like The GMR Co. spent so much time developing ways to eliminate it. Several of our members who track their cars have had issues.

http://thegmr.com/

Thank all of you for your replies.

The article I read simply said " we ditched the 12 bolt strait line Drag Strip rearend for a Currie 9 inch.
that could have been a marketing ploy from the magazine.

The 12 bolt is like new with 3.73 gears i bought it from a guy who never installed it. only bad thing is the drum brakes so i will be replacing them soon and using the 12 bolt thanks fellas.

On a 12 bolt with limited slip, a new unit will have no axle end play, a worn plate type will have some end play, re-shimming the clutch pack will reduce it. The Ford 9" has a better axle bearing but still has brake knock back problems. The cheapest solution is to use a floating caliper on the rear. If you want to use a fixed caliper, you need to install full floater axles.
David

Might have already been set up for drag racing ie: spool, numerically high ratio so it was problably a bit cheaper to go with the 9.

This guy knows what he's talking about. :thumbsup:

The 12 bolt can be built into a great assembly with the only potential issue being the axle retention. I would recommend having the housing end replaced with with an end that accepts a large Ford/Olds style 3.150"O.D. bearing or a floater assembly. There are C-clip Eliminator kits available but you will need to be sure to use one that has a tapered bearing vs the low friction ball bearing type. You've got alot to consider but it seems your getting some good information here.

Wil.

I have seen problems with knock back with both the Set 20 tapered bearings & the ball bearings. Currently I'm favoring the ball bearing slightly. The last rear we used I let Currie talk me into the Set 20 tapered bearing ends & we are not happy with them. The One Lap Camaro had the ball bearings & less knock back but did have a bearing failure.

So if building a 12 bolt (narrowing, buying axles and bearings) which setup is the best? Or do you just have to accept bearing failure, leaks and pad knock back? Is Going full floater the only real solution? But maybe it just depends more on the weight of the car, offset of the wheels and how hard its actually driven on and off the track as to how much problems you actually have.

When we make recommendations for axles, bearings and Diffs. we take all of the factors you listed into account.
Much like your suspension you will need to clearly outline the intended use of the car. Build the 12 bolt and enjoy the car. When things get more aggresive and the car is spending more time setting great lap times start to consider your options, floater assembly with new axles for the 12 bolt or a completely new 9" with a floater. Whatever you chose consider future proofing yourself.
Wil.