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!