As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT HEARD AND FELT ABRUPT CHANGE IN ENGINE 10 MINUTES INTO FLIGHT. UNEVENTFUL RETURN TO BASE WITH ENGINE INSTRUMENTS NORMAL. NR 4 CYLINDER HAD SHEARED ALL 8 STUDS AND HAD SEPARATED .50 INCH FROM CASE. ENGINE HAD HISTORY OF PROP STRIKE TEAR-DOWN 460 HRS PRIOR TO FAILURE.