As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE PERFORMING A COMPRESSION TEST NR 5 CYLINDER HAD AN AIR LEAK, SPRAYED SOAPY SOLUTION ON CYLINDER AND OBSERVED BUBBLES COMING FROM A CRACK BETWEEN COOLING FINS BETWEEN THE VALVES, UNDER THE UPPER SPARK PLUG AND ANOTHER AREA AFT OF THE SPARK PLUG HOLE.