Crank Thrusting Foreward Issues

FWIW..... You would be able to balance 318 rods and pistons with the 340 forged crank, even with the later, heavier 318 rods. The 340 pins and pistons are so much heavier than the 273/318 pins/pistons: the 340 bobweight is around 2326 grams, and the 318 (heavy rod) bobweight is around 2173 grams. So weight will go off.

I would also expect that the 340 cast crank would also balance out with the 318 parts, using the 318 neutral balanced damper and TC, but I have some data to gather to be sure that is the case. Keep in mind that 'external balanced' does not mean that all the counterbalance is external; actually, just a small part of the counterweights at each end of the crank is located externally. So since less total counterweight is needed with lighter rods/pistons/pins, it can be removed from the external parts first, and if enough is removed, then all of the external counterweighting can dissappear.