If I had to guess, which I do, I'd say this was an error made during assembly of the TB, or bad rivet(s). I'd guess the baffle came off relatively clean and got mangled trying to combust itself. Depending on the alloy aluminum melts at a relatively low temperature. It forges at 450°f, a third of the temperature where steel becomes malleable.

But no, it's most certainly not a common failure.

 

This wasn't a turbocharged engine was it?

 

Is the valve head bent? This should have shown up while lapping the valve as an uneven indication in the removed machinist bluing.

 

Dear all I have used a Neway Valve Seat Hand Cutter tool and remachined all exhaust valve seats. Since I am driving with propane all valve seats had wear near were the spark plug is.

I also replaced all exhaust valves and tested then by an reverse air leak test.

I took my mighty vac and with a vacumm special plate applied underpressure below from the cylinder head to test test.

So now I putting everything back together an I hope all works out Ok


Thank you all for support.

Greeting
Bob