ORIGINAL: aegis
Prior to mounting a muffler, I clean the threads on both the head and bolts really well with aerosal electronics cleaner, the type that isn't designed to leave an oliy residue. A little bit of thread locker and no problems.
Dan
Sometimes the muffler or the engine mating surfaces are not flat. Check for that with a strait edge. Sometimes you will need to square things up using a mill. Also some exhaust mufflers that have weight off to the side ...will need to have a brace added so viabration cannot set in. There are photos showing how this is done. Capt,n