I have done many homebuilt mufflers and the problem I find is vibration. The aluminum rod just doesn't have the strength. I have supported the exhaust pipe and this helps somewhat. My solution is to run the exhaust pipe all the way thru the muffler. Drill exit holes as needed in the exhaust pipe. This extra length and having the end of the exhaust pipe bearing on the wall of the muffler seems to do the job in most cases. I don't think heat is the problem,as the melting point of the rod is much higher than the engine will ever get to. Good luck, red