all the "reciprocating weight",(Pistons & rings, upper rod, etc.) and a certain percentage of the "rotating weight"(Rod big end and bearings) before getting a spin-up
I think you typoed that, it's all the "rotating weight", (Rod big end and bearings) and a certain percentage of the "reciprocating weight" (Pistons & rings, upper rod, etc.) before getting a spin-up.
An L6 is in theoretical balance because of the firing order.
However, the extra weight for both balance and inertial purposes is contained completely in the counterweights, but it’s simply not possible for the weight to be positioned directly opposite the component itself (it’s alongside for cylinders #3 & 4, but three cylinders away for # 2 & 5, and on opposite ends of the crankshaft for #1 & 6), so there is some flexing and bending force caused by the rotation of the eccentric weight as the crankshaft rotates that cannot be removed or fully compensated.
This isn't a matter of accuracy or quality control: it's not possible.
