rear differential question

[outback 1970]

I keep on going back and forth between the factory 12 bolt and a 9 inch,What do you guys think of the durability of the 12 bolt is? I have a fairly built up 402.I seems be doing fine right now but I am planning to go with a much wider tire out back.Is it worth putting money into narrowing the 12 bolt or just putting the money into buying and buildin a solid 9 inch?

[raycow]

If you are going to put that kind of money into a rear, go for the 9 inch. The 12 bolt will actually cost more to build than the 9 inch if you include the cost of C-clip eliminators, which the 9 inch doesn't need. Even after building the 12 bolt, you still won't have the convenience of quick ratio changes that the 9 inch gives you.

Ray

[btullis99]

I would have to say the 12 bolt in your truck is the best choice. a 12 bolt rear end is about 30 lbs lighter than a 9 inch. a 12 bolt loses 3.5% less power than a 9 inch. that is based off the rotating mass, and the fact the pinion gear is offset lower than a 9 inch. a properly set up 12 bolt will handle any amount of streetable power. sure the removable center section of a 9 inch is nice and all especially for the drag racing crowd. that's just my opinion, this debate has raged on for years with no end in sight.

[cdowns]

if you've got the gears you want and a good posi in it already i'd stick with the 12bolt
but if you need everything you should get the 9" built to specs . should b able to get a full built freash rear for right about $3000 ready to go

[Fitz]

Motorbooks.com has this manual, which shows in great detail just how to rebuild a 12 bolt diff. And, before someone points out that this only applies to 67 trucks, the diff is common to years before and after 67-72. You will need a 1/2 inch drive torque wrench and a dial indicator with a magnetic base and support arm. When I did my diff the gearset was about 270.00, the Eaton Posi 420.00, full install kit (bearings, shims, crush sleeve & gaskets) 88.00. C clip eliminators 160.00, new Strange axles 440.00 so the total was 1380 with new axles. Or just fork over 3 grand and have someone else do it for you. Your choice.

[cdowns]

Originally Posted by Fitz

Motorbooks.com has this manual, which shows in great detail just how to rebuild a 12 bolt diff. And, before someone points out that this only applies to 67 trucks, the diff is common to years before and after 67-72. You will need a 1/2 inch drive torque wrench and a dial indicator with a magnetic base and support arm. When I did my diff the gearset was about 270.00, the Eaton Posi 420.00, full install kit (bearings, shims, crush sleeve & gaskets) 88.00. C clip eliminators 160.00, new Strange axles 440.00 so the total was 1380 with new axles. Or just fork over 3 grand and have someone else do it for you. Your choice.

thats a good price cut narrowed with the new mounts and upgraded brakes for the big skins

[Davetopay]

Guys, keep in mind, the truck 12 bolt will NOT take the same power a car 12 bolt will stand up to. It's all in the pinion shaft diameter. If you make big HP with good traction in a heavy vehicle, they are known for shearing the pinion shaft off.

The 3.5% extra power loss is a bit of hyperbole as well. The deeper off set of the pinion gear does create more "wipe" against the ring, but it also increases contact patch, which makes it stronger. Not to mention the 3 bearings on the pinion vs 2.....

[dwcsr]

Originally Posted by btullis99

I would have to say the 12 bolt in your truck is the best choice. a 12 bolt rear end is about 30 lbs lighter than a 9 inch. a 12 bolt loses 3.5% less power than a 9 inch. that is based off the rotating mass, and the fact the pinion gear is offset lower than a 9 inch. a properly set up 12 bolt will handle any amount of streetable power. sure the removable center section of a 9 inch is nice and all especially for the drag racing crowd. that's just my opinion, this debate has raged on for years with no end in sight.

I agree.... .

[Fitz]

Originally Posted by Davetopay

Guys, keep in mind, the truck 12 bolt will NOT take the same power a car 12 bolt will stand up to. It's all in the pinion shaft diameter. If you make big HP with good traction in a heavy vehicle, they are known for shearing the pinion shaft off.

Big HP---ZZ572/620
Good Traction?---Hoosiers
Heavy Vehicle?---3300 # on the scales @ Western Waste Tucson with my fat ass in it.

Haven't broke it yet.

The pinion shaft will handle the power of a 401 with no trouble!

[btullis99]

the whole car vs truck 12 bolt pinion shaft diameter is irrelevant to me. the output shaft on my trans is 1.187 (I believe), I know its smaller than the pinion shaft, making it the weakest point.

[outback 1970]

Thank you everybody for your opions and facts,