As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING THE REPLACEMENT OF THE ENGINE'S EXHAUST RISERS, A MECHANIC NOTED THERE WAS A CRACK IN THE NR 4 CYLINDER HEAD THAT BEGAN AT THE EXHAUST STUD AND EXTENDED INWARD TO THE EXHAUST VALVE SEAT. FURTHER INSPECTION OF THE CYLINDER HEAD REVEALED CRACKING TO EITHER SIDE OF THE LOWER SPARK PLUG HOLE. AFTER THE CYLINDER WAS REMOVED, CRACKS WERE SEEN CONNECTING THE LOWER PLUG HOLE AND THE EXHAUST VALVE SEAT. THE CYLINDER WAS NOT DISCOLORED AS THOUGH IT HAD SEEN EXTREME HEAT, NOR DID IT HAVE ANY BROKEN OR BENT FINS OR ANY OTHER DAMAGE THAT WOULD CONTRIBUTE TO A CRACKING.