Also reference TSB 08-16-13. If Unison Ring Is Required For Repair; Refer To 3C3Z-6C885-A. If large Oring is required, use 4C3Z-00815-A, 13 Fin Center Housing Rotating Assembly (CHRA) kit. With bolt on style housing. For engine serial number 6,155,637 & up., BEFORE 10-1-2005