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

Strange, really Strange!!! Is your prop balanced? maybe some extra vibration could cause this

Yup, balanced, then rebalanced just to be sure. Another guy at our field had the same problem with the same engine in his RV-4.

There just has to be some vibration that's causing the heavy muffler/manifold assembly to move a bit. That will cause stress right at the engine. You'll need to support the muffler to prevent this from happening. With this large an engine, it's easy to have the manifold stressed. While you may not see much vibration, it's there, and it's causing the muffler to move around.