First, let me say I've never done this. Consider this discussion as a "brainstorming session". Everytime someone goes into their engine, there is a post telling them they should balance everything. This always leads to a discussion about "static" vs "dynamic" balancing. As I understand the movement of the rods in these engines, the big end moves in two axis while the little end only moves in one. If that is the case, then I can see where a simple weight balance could be misleading. It would be necessary to account for the vertical movement of the two big ends to attain balance. What if balancing were done with 2 scales? By suspending each rod horizontally from two scales suspended from a horizontal beam and matching the weight of each rod on each scale, wouldn't that account for movement in both axis?