What was found
(CAN) PRF MESSAGE CHECK ENGINE NR 2 FIRE LOOP "B". THIS DEFECT WAS UNDER INVESTIGATION BY MAINTENANCE PERSONNEL AND ALSO DEFFERED UNDER MEL 26-12-01 CATEGORY "C" AS A RESULT OF THIS PFR MESSAGE. FIRE LOOP "B" REPLACEMENT WAS SCHEDULED WHILE OPERATING THE "C" DUCT FOR LOOP REPLACEMENT IT WAS NOTICED FOR DAMAGE TO THE NR 2 ENGINE RT "C" DUCT HEAT SHIELD. THE DAMAGE HAS BEEN CAUSE BY A MISSING "PISTON RING" (P/N336-510-710-0 OR 649-395-073-0 POST SB).MAINTENANCE PERSONNEL INSTALLED NEW PISTON RING, BUT AIRCRAFT WAS GROUNDED DUE TO THE EXTENSIVE DAMAGE TO THE 32 ENGINE RT "C" DUCT HEAT SHIELD. A TEMPORARY RE PAIR HAS BEEN ACCOMPLISHED TO THE HEAT SHIELD WITH A REPETITIVE INSPECTION EVERY "A" CHECK GRANTED BY BF GOO