the OEM pulley is indeed a harmonic balancer. Flip it over, it is 2 pieces separated by a rubber elastomer layer. It is tuned to a specific resonance frequency and is designed to dampen it. This is what you give up when you run the aftermarket pulley. But hey, I haven't run into any problems yet w/my ASP pulley, and I love the increase in engine RPM response (both up and down) as well as the power boost. Once again, the OEM pulley LOOKS LIKE it is 1 piece from the front side, but I believe it is actually two piece.