I'm just gonna throw in a couple things that I think I picked up while having mine balanced. I think its about having equal crank halves to start. Installing the two halves correctly is important too. The rod and piston are too far away from center to ever get it to be perfectly balanced, but engine builders are able to get it balanced in a certain rpm range. Modifications that lighten the piston and rod help the "balance." Also alot of cranks are not true in their circles that they make while spinning. If either half of the crank is bad on it tolerances, I don't think you can fix it. I'm no machinist, and could not grasp everything that was explained to me, but I did understand that it is really tough to get there without good matching crank halves.