Just a thought: the well-known "perfect inertial balance" of an L6 is not a physical analysis, but theoretical.
In practice, each pin and its components is opposed by another cylinder on an opposing stroke (unlike L4, V8, etc.).
Where's the difference?
The other cylinder is (at best) next to it (not exactly opposite) such as the #3 & 4 (the balanced #1 & 6 are at opposite ends, duh), and the forces between them bend the crank at the web connecting them. The counterweight provides opposing weight as close as possible to the actual pin (on both sides).
But, again, what does it weigh, and why?
