the crank shaft its harmonic balance?

Originally Posted by Trilitheum

Just to add some more data into the debate, here is the US patent for a plastic hub torsional harmonic damper which has some good graphs that illustrate the effect on crankshaft twist in degrees vs engine RPM for various damper configurations including no damper.

Even if the engine is perfectly balanced there will always be torsional pulses which can and do form harmonic modes as a result of the discrete, not continuous combustion pulses (each cylinder firing in turn). Balancing the entire rotating assembly will not stop rotational twist of the shaft from the power pulse as each cylinder fires. It will stop out of axis oscillation.

The pulley being made out of cast iron is also on purpose, cast iron has one of the highest dampening capacities (google cast iron damping) of any metal.

If you don't want to read the whole document look at Figure 2 to see the effect on torrosional vibration via a typical damper (construction shown in Figure 1)