It is pretty common on these trucks for one frame rail to have been pulled on, causing the frame to be out of square. The frames have no center X-members, only riveted in cross members, and have very little resistance to being pushed or pulled out of square. Typically this would've happened when the truck was stuck in the mud in a field or something, and someone hooked a chain to the most convenient spot (spring shackle on one side), and gave it a jerk or two or three with the tractor. This pulls one rail ahead of the other causing the out of square (parallelogram or diamond shaped) condition. When the frame rails aren't square to each other, it shows up as the torque tube not locating on the center of the rear of the transmission.
Usually this shows up after the trucks suspension has been rebuilt, as when everything (rear end saddles and spring eye bushings) are worn out, there is enough slop to allow the torque tube to move on center. When everything is new and tight, it is where it is. So basically, if one rail is ahead of the other, the rear end is not sitting square in the frame, and the amount of this error is multiplied by the length of the torque tube.
An easy way to check your frame for squareness is to simply measure diagonally from common points on the frame from left front to right rear, and visa versa. Use common holes or rivets as your reference points. Pick points as far apart as possible. (Very front cross member rivets and very rear cross member rivets are good) The diagonal measurements should be within 1/4" of each other, an 1/8" is even better. If the cab is on the frame, and it's hard to measure, you can also use a plumb bob to extend your reference points down to the garage floor and mark them out there with chalk or a Sharpie marker. Then you can check your diagonals right on the floor. If you determine your rails are out of square, it's easy to imagine which rail needs to go forward (or back) to get the diagonals the same, and then imagine how that would effect your torque tube alignment.
I have used a Porto-power (bodyman's hydraulic jacking tool) or come-a-longs to push or pull on the frame rails diagonally and get them back to square. If you get it square, and it will stay there on it's own, bolt the torque tube up and you are probably fine. I have gone so far as to weld cross members in, and even add small gussets to the cross members to keep them square. If the rear end is in the truck, but not bolted to the transmission, you will actually see the front of the torque tube move towards the center when you push or pull diagonally on the frame in the correct direction.
All this sounds harder than it is. If you have a bare frame, 15 minutes with a tape measure will tell you everything you need to know. It's not uncommon at all to have an old Chevy truck frame be out of square, and it's easy to check when the truck is apart.
Last edited by Barry Weeks; 04/24/2012 4:56 PM.
