Originally Posted by the above thread
If you want to get fussy, and allow for the port being moved, balance on a "wheel" type balancer. This can even help on a stock type crank. (see some of the Russian engines, .15 size for examples). The amount of counterbalance needs to be in the 50~65% range of the weight of the piston,wrist-pin,(ring, if used) and 1/2 the weight of the rod. To test this, measure the weight of the reciprocating parts on a good gram scale, and make up bob-weights in this amount to add to the crankpin, before you put it on the balancer.The crankpin should "stop" at the 12 O'clock position. On a single cylinder engine: If you add weight to the counterbalance, or lighten the reciprocating weight. you DECREASE the PRIMARY shaking imbalance (vertical= Cyl. bore axis), and INCREASE the SECONDARY imbalance (crosswise= across the lugs).
For this reason, IT'S IMPOSSIBLE to balance a single Cylinder engine, without a secondary Balance shaft, as used in some auto engines. Adding weights (heavy slugs) to the counterbalance is done to change the resonant frequency at which you get a "Harmonic" or strong vibration level. You want this at a lower than full rpm number. Most of the ST X-40's used in Formula-1 Pylon and Fast Rat had cranks that needed this changed to resonate in the high-teens (RPM). Sometimes you could see the engine "shake" in this range, on the test stand. I never put a strobe light on one to see what one would look like while running. If the engine is balanced to "resonate" at full RPM, IT USUALLY COMES APART