I have one OS 55 AX and one 75 AX, both engines work superbly, no troubles and lots of power. The power box mufflers are quiet too! On the last flights in the fall i noticed that the muffler on the 75 started to sweat, and a closer inspection revealed that the centrescrew had started to loosen. One should anticipate that you just tighten the nut, and then the sweatting would stop, but in fact, I had to turn the nut the opposite way to get it tight. Can anyone explain this?