Most crank pulleys are a 2 piece unit glued together with rubber. Through the years the rubber rots and one piece can shift on the other causing a vibration and a timing mark shift. This causes a multitude of problems which can usually be cured by one of our rebuilds.
The core you send us has to be free of cracks, chips, or metal damage to get a core charge refund. Our rebuilder uses silicone rubber and goes to great lengths to make sure that these pulleys are better than when they were new.