I have a 5 month old FS-91 engine in which the the muffler came loose on the header pipe. I had the locknut on the header pipe tightened down on the muffler securely. The muffler must have loosened up and the threads on the inside were destroyed by vibration. I didn't catch the loose muffler because it is behind a cowl. I cant re-thread the muffler because so much of the threads are gone. The threads on the header pipe are ok though. Is this a problem with the muffler or is there any way to secure the muffler better? If the mufflers weren't so expensive it wouldn't bother me as much but I would hate to loose another muffler due to the same problem.

I have since bought a FS-120 and after the first few flights the muffler came loose on it as well, but the engine is exposed so I caught it right away.

Both the engines run great by the way.

Thanks for any help!

The reason the mufflers can become loose is because you have a heavy muffler on the end of a long moment arm. As the engine vibrates, the muffler is moved around at the end of the header. If you secure the muffler to the airframe, the problem goes away.

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.

It would be appreciated if Mr. Baxter would advise as to most appropriate manner in which to secure muffler for FS 9II Surpass to the airframe, especially with cowled engine (i.e UCANDO 60).
Kindest regards,
Chepenik