I encountered the same problem with an OS 91 FS w/pump and solved it with an application of Permatex High Temerature Thread sealant on the muffler threads. The threads stripped the first time I flew with the motor, then I saw the $50 price tag for a new muffler. I used this thread sealant on the original stripped muffler and now, two years later, I am still using the same muffler, the threads hold tight, the muffler does not loosen, and the connection is sealed. Allow 72 hours after application before running the motor.