12 bolt rear question

[jb4short]

I've got a '68 12 bolt rear (6 lug) and a potential issue with alignment with the rest of the drivetrain. The center of the pinion is offset 1" towards passengers side; is this the way it should be? Do you angle the motor and trans to line up with it that way? I'm worried about the driveshaft being too close to the ECE crossmember

[Longhorn Man]

that's how they were built.
This is to ensure the U joints are turned a little bit regardless of how loaded it is. If you roll with it loaded to max capacity all the time (farmers/lawn care guys/scrappers) then the U joint would not be angled up and down, so they offset it so the bearings would not stay in the same place.
If they don't move around, they dig into the U joint metal causing failure pretty quick.

[jb4short]

Originally Posted by Longhorn Man

that's how they were built.
This is to ensure the U joints are turned a little bit regardless of how loaded it is. If you roll with it loaded to max capacity all the time (farmers/lawn care guys/scrappers) then the U joint would not be angled up and down, so they offset it so the bearings would not stay in the same place.
If they don't move around, they dig into the U joint metal causing failure pretty quick.

Thanks....just doesn't look right...LOL

[batteryguy]

I have the same issue with my '67 short narrow bed pickup. It has a '71 leaf spring frame under it. I am going to have the rearend narrowed to get the pinion right in the center of the frame rails. The pinion angle will then be set by rotating the pinion down to achieve the right angle. Narrowing the rearend will also allow me to put on much larger tires.

[Longhorn Man]

hey guy, you better talk to a drive line shop, or a shop that sets up prostreet suspentions. Altering the pinion angle chainges the way the truck would hook up.
It is offset for a reason.

[dwcsr]

I had my rear narrowed and pinion centered and had to re-engineer the Tri-bar setup to work with it and I had to have axles made. It was costly and a PIA.

The original intent by GM for an offset rear was to save on axle manufacturing. One axle fits both sides so less machine tooling and fewer inventories saved them big bucks.

Center the engine as you would a pinion centered rear. The advantage of centering the pinion is you get rid of the compound angle in the driveline. If you’re racing that becomes an issue. If not I'd leave it offset. It’s worked fine for however many years it’s been in use and you save the expense of axles and chopping the rear. Take a look at this link it may be helpful in lining up your drive train.

http://www.thehollisterroadcompany.com/framesquare.html

[batteryguy]

I have consulted a race car shop and centering the pinion was their recommendation. I do expect to take the truck to the dragstrip every now and then. It makes 450 horsepower and 470 lb ft of torque now. I plan to add some sort of power adder later to increase to 550-600 horsepower. I don't want to have any problems with the driveshaft so I want to get it straight now. I am very early into this project so making changes now is easy.