I have a new 91 Surpass (non-pumped). After running a tank through, I have 5 flights and have now gone through my 3rd header (pipe between engine and muffler). Each one has cracked right on the threads in the muffler. This latest time i checked and double checked that the lock nut was tight (but not too tight) and there was no vibration when I took off. Any suggestions? I can't believe it's so sensitive to movement. No other engine has had this problem.

thanks.

Bob P