I had those bolts sheer off on my '50 1/2 ton, and ended up making the joint go cock-eyed and blew out the side of the tranny. I really drove the balls off that truck, tho.. dragging a 4,000 pound car up a hill on it's roof with a stock 216 and a 3 speed probably wasn't what the truck was built to do.. but it did it.
The '46 1/2 ton I just got had the same thing happen to a previous owner.. the u-joint was split when it arrived, with two of the bolts sheered off and the other two missing.
I ended up drilling all four holes out and tapped them for bigger bolts, with a little less fine a thread, cranked 'em in tight with some loc-tite, then folded over the tabs to keep 'em from unscrewing.
My guess on why these joints fail is that one or two of the bolts end up loosning up, making the joint sloppy.. with one bolt out it isn't long before the wobbling ends up working the others loose, or sheering them off.
