If you're talking 55-64 type Chevy, then that's not too unusual, especially if you have lots of power and lots of bite in a heavy car. The main caps are the weak link and need to be reinforced with machined caps or straps which also keep the main bolts from spreading apart. The caps usually break apart right where the adjuster lock bolts go. Waaaayyy back when I was racing a 57 Chevy, I had my local machine shop fabricate a pair of straps that fit on top of the factory caps that had a crush of .003 . They also machined flat spots on the factory caps for the new caps to press on. The new machined steel caps were drilled and tapped on the side to accomodate the modified adjuster locks. I had to use longer allen bolts to hold the whole affair together and had to cut the rear cover off and weld in a piece of one inch strap to space the cover back for clearance, but I never broke another rear end. I used to come off the line at 6000 with a 4.88 rear end, 40 pound flywheel, Schiefer Rev Lock clutch, slicks, Summers Brothers axles, traction bars and all. The front tires were six inches up on most tracks. Those puny rear ends used positraction units made by Dana and side gears out of some type of 60's Mopar could be substituted so larger and stronger axles could be used. Now all you need is a 60's Car Craft or Popular Hot Rod stocker magazine "How To" article, or get on the phone Monday to some of the more knowledgable rear end guys.
