As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CYLINDER WAS CRACKED FROM SPARK PLUG HOLE TO SPARK PLUG HOLE ON THE EXHAUST SIDE. THE CRACK HAD OPENED UP TO A SIGNIFICANT SIZE HOLE MAKING THE EXHAUST VALVE VISIBLE FROM BETWEEN THE FINS. THE LEAKING EXHAUST GASES HAD BURNED AWAY A PORTION OF THE FINS AROUND THE OPENING.