As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING ROUTINE MAINTENANCE, METAL FOUND IN OIL FILTER. TRACED TO BROKEN INNER VALVE SPRING OF NR 5 CYLINDER. BROKEN ENDS HAD STARTED TO EAT INTO CYLINDER HEAD. SUBMITTER SUGGESTED CAUSE AS METALLURIGICAL FAILURE OF SPRING STEEL. VALVE SPRINGS ARE A MANDATORY REPLACEMENT ITEM AT OVERHAUL (OR RE-MANUFACTURE). THEREFORE, THIS PART HAD ONLY ACHIEVED ABOUT HALF OF ITS RATED LIFE. THE OUTER SPRING WAS INTACT.