I'm running a Xoar 18x8, engine was already broken in some, I finished the break in. Peak is 7150 rpm set slightly rich. Could be too much prop, I have considered that as possibly contributing to the excessive heat. Retarded timing can do the same thing, finishing the burn too far down the cylinder wall. The total oil content of my fuel is 17.5%.

If this engine can't turn a 18x8 I'll be getting rid of it. 16x8 is not going to do, a 17x8 might be ok.