How so, Markus? Are you assuming that the opposing rods, pistons, and even crankshaft parts all weigh exactly the same? They don't. As long as they're close, the engine should be fairly well balanced. But unless they're identical, the engine will vibrate.
edit - didn't see "balance shafts" and assumed you were talking about harmonic dampeners. Oops
edit again - Jewbaru, sorry 'bout the mistake. What engines still use balance shafts?
[This message has been edited by Joel Gat, 1.8L (edited October 02, 2000).]