I have been flying R/C since 1972. This means nothing but does give a frame of reference. It is my opinion that when the muffler center bolt breaks in flight, it is because the muffler halves and center bolt, and maybe the muffler and the engine, have been rattling around beating each other up, generating greater stresses than if everything was tight and held together. This is why I RTV every joint and loctite the bolts, and do the job over whenever I see a sign of a leak. I would not be surprised to learn that the muffler center bolt is made of an alloy steel with a tensile strength well over 120,00 PSI and the replacement all thread rods are made of low carbon steel with a tensile strength of around 60,00 PSI. If you lightly hammer that center bolt at 12,000 blows per minute, is it a surprise to see it fail? If things are kept tight the bolt will works like a spring and absorbs the 12,000 blows per minute because the stresses will be within the bolt elastic limit. I almost lost a muffler about 10 years ago. Then I started to use the RTV and loctite.