OK....I have to jump in on this again. I have only used Saito and O.S. four stroke engines in all my models. Never had a muffler or extension come loose. I always clean the threads (all threads, inside and out) with any good cleaner. Then apply a small amount of regular motor oil. Lock everything down real tight and then run the engine. Make sure the prop and spinner are balanced and only use a metal motor mount. Get rid of the black plastic ones away. Run the engine through a full tank of fuel and then let everything cool down. This has work for me going on 20 years now.