As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING AIRCRAFT RUN-UP, THE PILOT NOTICED THE ENGINE WAS RUNNING ROUGH. THE MECHANIC PULLED A COMPRESSION CHECK AND FOUND THAT NR 3 CYLINDER HAD NO COMPRESSION. FURTHER INVESTIGATING AND CYLINDER REMOVAL SHOWED THE EXHAUST VALVE SEAT INSIDE THE CYLINDER HAD SLIPPED OUT FROM ITS GROOVE; THUS, NOT ALLOWING FOR PROPER VALVE SEATING.