Good on you Raven for applying the scientific method to test your theory, even if it is just a mind experiment. Do us a favour next time though, and write down what you observe. That way, we can see if for a significantly large number of engines, that placing the prop in line with TDC agrees with your hypothesis. Going off memory is not the best way of doing it, and I doubt you'll find anyone else to carry out this work but you. I'd say go for it and report back here with what you find.

I'd like to try and help you get a grasp of this though, even it it's just so that I can prove to myself that five years of Uni was not a waste of time, (including the time spent on an undergrad thesis on dynamic engine balancing) so here goes:

You are right, there are many vibrations in any reciprocating engine that are for practical reasons, impossible to balance out, and a two cycle single is one of the hardest of all. The fact that the prop does not have it's mass evenly distributed in a disc about it's point of rotation though is not one of them. Yes it's mass is mostly distributed along a line which intersects the prop point of rotation and the prop's own center of mass, but each blade is balanced statically, dynamically and aerodynamically (mostly) by the other, and therefore exerts no "sinusoidal" load on the crank. There is no force going into the hub from the prop that changes as the prop rotates. The same is true regardless of how many blades the prop has. Think of a multi bladed fan as in a turbine engine. Does this fan exhibit a vibration equal to the frequency of the blades passing some arbitrary point, purely due to the spinning mass?

m.