In my experience, the centre bolt was tight. The muffler joints were RTV'd. It had not loosened in 20+ flights, and in fact, the nut and the bulk of the bolt were still in the back half of the muffler, still tight. The head itself had popped off, a tensile failure.

Aluminum expands at about 2x the rate of steel with heat. I think I probably had slightly over-tightened that centre bolt, and when the muffler came up to heat, there wasn't enough stretch left in the bolt. There was no sign of necking down or other plastic failure modes, just a sharp, right angle brittle fracture. Either I over-tightened the bolt, or it was improperly heat treated. It should not have failed at that location, there's even a fillet under the head to prevent this stress rizer. It should have failed in the threaded portion.

I've wondered if I should thread the ends of a piece of music wire, then bend a little zig-zag in it's middle, harden and temper it (I have that technology), and use that for the thru bolt. Music wire is fine high carbon steel, huge tensile strength, and a little zig-zag would give it muchos stretch, while still holding the muff together. And glow engines exhaust doesn't get hot enough to ruin the temper...

Maybe I'll try that if the current arrangement fails again....

J