Hi Bill, I think that 3.5 inches in diameter is close for a lot of 2 cycle engines. That is what a Toro is and my McCulloch engines are. After all, Walbro and Phelon make a lot of these as suppliers. While we are on the subject of a flywheel helping to smooth out the non-linear power pulses of a single cylinder engine, I happen to remember where you can actually see the power pulses of an engine. I have seen this at an airport and most recently on Wings on HBO, where they have shown a big Radial engine idling. Ever notice that the blades of the prop on a big recip seem to play with the Sunlight? If you look closely, at the slow idling speed, you can actually see the varying acceleration of the propeller tips as the cylinders fire. I had noticed this many many years ago and then later after I had learned how an engine ran, I became more observant. It is noticeable with the big recips and their 3 or 4 bladed props and about the only place to see them now is on Wings. I bet the mechanics on these engines noticed it and just took it for granted. Another thing I remembered was in an RC magazine about a year ago where a big twin gasser sheared the engine hub off behind the prop at full throttle on a ground run up. No one was in front of the engine and the prop stuck a blade in the ground breaking one blade of the Polycarbonite prop off. I would suspect a harmonic problem of some kind here. This was a 30" prop as I remember. I have propped full scale airplanes a few times and I make it a point to respect all props.