Value in 10 bolt to truck 12 bolt swap?

[nikwho]

So, I've read that the truck 12 bolt rear axle's have the same pinion diameter as the 10 bolt rear axles, and that they are smaller than the car 12 bolt pinions, as well. I have a friend that will trade his 1980 C10 12 bolt rear end for my 1986 C10 10 bolt rear. I am building a '68 GMC that will have a healthy 600HP BBC. I plan to drag race it once or twice a year, enter a couple of autocross races per year and primarily drive the truck on the street. Most trucks are a little easier on rear ends than cars, due to the f/r weight distribution. So, right now I'm trying to decide if It's even worth the time to swap a 12 bolt truck in place of the 10 bolt. I figured that if the pinion shaft diameter is the weak link, is it worth having the additional unsprung weight of a 12 bolt axle, or should I just stick to my 10 bolt and build it?

I've got about a 2,600-2,800 stall converter and will be leaving using just a foot brake, no trans brake. Also, I will be running some 27x11" Goodyear slicks, with 4.10 gears.

Thanks!
Nik

[PGSigns]

Well I would do the 12 bolt. For what you plan to do while it is out have c-clip eliminators put in it, a Eaton posi and the gear you want along with a set of long wheel studs. This will get it solid and pass the drag race rule when you go. Or just build a 9" and have the ability to swap from a drag gear to an auto cross to a street gear with just a pumpkin swap.
Jimmy

[GASoline71]

I beat the p1ss out of my 12-bolt truck rear end with an Eaton Posi-traction unit. I have yet to have a 12-bolt truck pinion fail. I've busted a couple of ring gear teeth... but never a problem with the pinion.

Gary

[mongocanfly]

id probably say 9" as well....

[nikwho]

I've been leaning towards the 9" as well. In a 13 bolt truck housing (not the stronger car 12 bolt housings) what advantage does a 12 bolt have over a 10 bolt? I think that I read that the pinion shaft diameter is either the same, or withing 0.001", IIRC. is that really the weak link? Either way, IF I stuck with a GM housing, I'd likely swap to aftermarket chromoly axle shafts and would have to do c-clip eliminators, as mentioned above. The c clips are are one of the factors, along with 3rd member changeability and the ability to weld on the housing, being that they are not cast iron!

[Captainfab]

The 10 bolt has a 8.5" diameter ring gear as opposed to the 12 bolt which has a 8.875" diameter ring gear.

[nikwho]

Is that an advantage with the pinion shafts being equal? I've always thought the swap to be a no brainer. Though, hearing that the pinion shafts the weak link and them being equal, is it really tougher? Where it will actually break?

[PGSigns]

We ran mud bog trucks for several years with 12 bolts and never broke a pinion. We broke a bunch of gears and twisted some splines but never a pinion. None of the truck 12 bolt is cast iron but the center is cast steel. We have welded the tubes to the center a couple of times no issues with proper welding procedures. So no issues with welding. Now gears come in different flavors. You can get drag race gears, off road and street gears and the low price gears. So application drives the gear material. So a well built 12 bolt will serve you well, but with your plans to drag race, auto cross and street drive I would build a 9". Then there is the suspension ad how you decide to do that as all three of your choices can drive 3 different suspension setups. A good auto cross setup will not make a good drag race one.
Jimmy

[nikwho]

Good to know! I will need to figure out if I want to build a 9", or go with the 12 bolt. I'm pretty certain that km going to build another 9". Speaking of event/driving style specific suspension, I built a set up for my '53 GMC with multi-purpose driving in mind. I worked on the geometry for a long time, actually a few months before cutting any tubing or brackets. I actually worked out the geometry of two seperate suspension systems that each had the same lower link mounts. Each mount, both frame and axle sides had vertical adjustability, so they weren't to be set up identcally, but very close.

It's a triangulated three link and a parallel 4-link that each utilized the same lower link bars and mounts, but each had different link bars and mounting positions for the upper mounts. I can pretty quickly and easily switch between the two setups. I like the three link better, as it centers the axle and does not require a panhard bar or watts link. I want to primarily autocross the truck, but have the ability to take it down the drag strip occasionally, too. I may build something very similar for this truck, but have been kicking around a satchel link setup for this truck. It's like a backwards triangulated four link or a three link.

For my '53 GMC, I narrowed a 9" out of an Econoline van. I had bought a turned, ground and polished shaft and turned some mock up bearing blocks on the lathe to mount in place of the carrier bearings, as well as the axle ends to verify that each tube was lined up with the carrier. Worked very well. Anyway, got off on a tangent, but yes, I'm always thinking of different suspension setups for the way I like to drive my truck. I've enjoyed building the 9" housings that I have narrowed. I will likely just stick with building another. I tried to rotate this picture twice of my three linked '53 GMC, but it wanted to stay rotated. Sorry.

[nikwho]

In that picture you can see that if you remove the upper three link bar, there are provisions for seperate upper parallel four link bars that each attach to the same axle brackets the lower link bars and coil overs attach to. This picture was taken with the three link mocked up, prior to the panhard bar, that later got scrapped when I decided to go with a watts link.