I am putting togather another RYOBI engine, and have had it running. This block is one with two slots on each side, a double thin ring, and a somewhat oval shaped exhaust port (no center post). It has the heavy duty connecting rod as well. I have raised the exhaust port to 140 degrees, and squared out the corners. A 26 carb and custom muffler and EI set at 30 degrees with an auto advance. I was going to leave the gasket out, but there is not enough head room unless I bevel the piston. Presently it will turn a 18X10 wood "TOPFLIGHT" prop 8000 RPM. My question is "What is the maxium exhaust porting possible without ruining the set-up?" Has anyone had this experience? Thanks. Flathead 4