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Q. I have been working on my '56 F-100 cab for over a year. I have replaced most of the floor, the bottom of the cab back, one rear cab corner, both rocker panels, and both front cab corners. As I worked on the cab I learned the truck was hit pretty hard before I got it. The impact wrinkled the firewall on the passenger side, and I see signs of the impact on the framerail at the rear cab mount. I have worked out all of the damage in the firewall, and I have measured the frame in an X pattern to make sure it is square.
The problem is that the cab was about an inch lower in the back on the passenger side. In the front, I measured from the top of the frame to the top of the firewall, and it was the same on both sides. So, I placed a 1/8-inch shim between the frame and the rear cab mount, and this raised the cab about a half inch. Then I installed some two-way adjustable rear cab mounts, thinking this would solve my problem. After removing the old rear mounts and removing the bolts from the front mounts, the truck cab took on a twist, so now one cab corner is higher than the other three corners. Everything was bolted down when I welded the floor and corners together.
Should I just jack the cab around, then bolt it down and ignore what is happening, or should I correct the twist? This might require cutting some of the floor loose, re-bolting all four corners, and re-welding the floor. What is the best way to measure the cab to see if it is square?
Via the Internet
A. It's tricky to perfectly straighten a cab in such condition. As you are well aware, each panel that you welded into place is helping to keep the cab twisted, and it's not very likely that addressing one of the two panels will release the twist you have-you'll most likely have to cut many (maybe most) of the welds you have painstakingly made.
So, given that it's a very big job to correct the situation, I'd suggest taking a long and careful look at exactly what problems would result from allowing the cab to retain the twist. If the fit of the doors is greatly compromised, then I'd suggest "biting the bullet" and getting the correct alignment. If the fit of the cab to the bed is so different from one side to the other that a casual observer would notice a problem, then I'd probably suggest making corrections for that reason, too.
But if the doors fit pretty well, and you can't really see any problems with alignment of the body panels, you could build a pretty strong case for allowing the discrepancies to remain, and only telling your closest friends about the misalignment, since it's not likely anyone else would know unless you told them.
If you do decide to make everything right, I'd start with the frame, which you mentioned showed signs of impact. I don't know if you have straightened the frame, but you'll need an accurate base to build from if you want to get the twist out of the cab, and straightening the frame is the best place to start. You said the X dimensions are true, but the frame could still be bent or twisted, even so. Once you know the frame is straight, then you can bolt the cab into place, and measure everything, using the frame as a reference to find exactly where the discrepancies are. You can do a little tweaking by using shims under the cab, but there is a limit to how much you can untwist the cab by cranking down on the mounting bolts. Once you've identified exactly where the problems are, I'd start cutting the inner structure wherever necessary to allow the cab to be untwisted. It will probably be necessary to use a Porta-Power, or perhaps a bottle jack to make the cab move. It's possible you'll change the fit of the doors in doing this jacking around with the cab, which will most likely require further corrections. But with enough careful measuring and adjusting, you can bring the cab back to whatever tolerance you want.