Depends on how many times it has already happened. A close inspection of the cylinder thread bosses is in order to determine if they have been damaged by a loose mounted muffler. You can get away with it once, usually, but if the muffler is not well secured after that the previous damage is expanded, with each following event making things worse. It gets to a point where correcting the damage will be the only way to keep a muffler on. Helicoils, tapping to the next laeger,, or two sizes larger screw, or cylinder replacement.