As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING INITIAL CLIMB, THE CREW EXPERIENCED A SIGNIFICANT POWER LOSS. THE INSTRUCTOR PILOT LANDED THE AIRPLANE ON AN ALTERNATE RUNWAY. INSPECTION REVEALED A CRACK IN THE NR 2 CYLINDER HEAD ORIGINATING FROM THE INTAKE VALVE SEAT AND CONTINUING THROUGH THE CYLINDER TO THE OUTSIDE BETWEEN THE COOLING FINS. A COMPRESSION CHECK REVEALED A LARGE AMOUNT OF AIR EXITING THROUGH THIS CRACK MAKING AN AUDIBLE HISSING.