self loosening exhaust bolts? Options

[lincoln]

second time its done this- car is fine driving to work but come time to leave , the bolts at the head have come loose. i DID crawl under and from what i saw the idiot who rebuilt the motor stripped about half the bolts out so are only half of them still there. i tightened em up- whats left - and it was ok , but it just did it again and im wondering if anyone else has come across this problem.

i am likely going to be ripping out the exhaust and re-tapping all the bolts up a size or so , which means also re-drilling the pipes for the new sizes . hopefully that fixes the problem.

thanks in advance ,
Lincoln