Also reference TSB 08-16-13. If Unison Ring Is Required For Repair; Refer To 3C3Z-6C885-A. If large Oring is required, use 3C3Z-00815-A, 10 Fin Center Housing Rotating Assembly (CHRA) kit. With bolt on style housing. For engine serial number 6,000,000 to 6,155,636.