Mattax said

I think its more about the chances of running it lean and advanced during break in are higher, The friction idea is possible. Another is the rings aren't fully sealing at break in. I don't know why that would drive the EGT up but maybe it can.

Bottom line is most header coatings start to fail under high exhaust gas temperatures. Might be worse if they haven't fully cured - that part depends on the coating and the process.

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