I've been watching ,reading, learning, and remembering this thread. To some of you, this thread may not mean much but there has been a couple of real knowledgeable guys share their views here that others should take note of.

The only thing I would like to add or repeat is that Clarence Lee once said not long ago and that was to try to balance a single cylinder engine is only a compromise.

The thought, I have been having, while reading the part about the ST3000 is, there needs to be a dynamic balancer available whereby an engine could be observed at what crank angles the strongest vibration occurs so that weight could be added or removed to smooth the engine out. This gets back to someone's statement about a Trial And Error Process .

The automobile companies resorted to balance shafts that opposed the vibrations in 4 cylinder car engines. There were natural harmonics formed in these engines at certain rpms that could not be balanced by counter weights on the crank alone. Yet the "A" model Fords had no counter balances at all.

Another important item that is noteworthy is that a vibration of a certain magnitude is less evident at a higher rpm than at a lessor rpm. Therefore larger props can cause more vibration because the engine will run slower? Yet a model engine will idle at a lower speed because of the flywheel effect of a larger prop. Also, this flywheel effect, will serve to smooth out the power pulses and and, therefore, make less vibration?

I'm also an old 'hot rod' guy and I always thought you weighed everything seperately and took all the weight of the big end and half the weight of everything else. But yet again, that was before many toughts of natural order harmonics came along. The problem with natural harmonics is that everything has a natural frequency whereby it will vibrate. There are times that another part in the engine will have a frequency that is a 3rd or 5th order harmonic of the first one and these will amplify each other.

In recent years, there has been a lot of thought about harmonics. They are doing wonders with it on auto bodies in recent years thanks to the computer.

I remember seeing a documentary on the making of a late model jet engine (CF-7?) where in the end they ran it while observing oil lines etc to see if one vibrated. If they found one, they would move the bracket over a little to change the natural hoarmonic frequency of that piece of tubing so it didn't harmonize with the other vibrations.

Trial and error?

Jim