As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
INSP FOUND THE LOCKING PORTION OF THE INSIDE DIAMETER OF PN 21361 (KEEPER) WAS STRIPPED AWAY ALLOWING EXHAUST VALVE TO ENTER COMBUSTION CHAMBER OF CYL NR 1 DURING FLIGHT. KEEPERS ARE OBVIOUSLY OF LESS HARDNESS THAN VALVE STEM. TIME IN SERVICE, OSCILLATORY FORCES, AND POSSIBLY DETONATION PEAK PRESSURES MAY HAVE RESULTED IN FAILURE. PERHAPS PART HARDNESS WAS INSUFFICIENT.