What prop are you running? 10k at half throttle? What is your RPM at WOT?

I've never used thread locker on the muffler, the AC has a straight muffler that is threads directly into the head and I just tighten it. The 91FZ has a pipe that goes from the head to the muffler and it has lock nuts on either end..again never needed thread locker so I can't in good conscience make a suggestion.