Most of our rebuild items are sold exchange, and you are required to return your old unit. In order to insure that we have all cores returned in a timely manner, we charge a core charge until the old unit is returned. You have 30 days to return the old unit. All cores must be complete and "as is" off the engine. Furthermore, they cannot have been in a fire or damaged in an accident. Other than that, all cores charges are fully refundable. If you desire, you can send your old unit prior to shipment of you order, and the core charge will not be charge.