Dave,

My problem is not due to overtightening and I did "lock" the threads on the new muffler screw. The screw is stretching under the head from the muffler movement. The combination of the HOT exhaust hitting it at this point and the leverage from the long muffler working it is the problem. You can see with a magnifying glass the shank discoloring and stretching about 1/4" long just under the head. Your rivet idea is a good solution. It locks the three pieces together and transfers the load from the screw to the muffler castings. Was there enough material in the two castings "ledges" to drill through and have a good uniform hole to upset the blind rivet?

Regards,