As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT DECLARED AN EMERGENCY SHORTLY AFTER TAKEOFF DUE TO ROUGH RUNNING ENGINE. FOUND NR 4 CYLINDER HAD .50 INCH DIAMETER PIECE OF ALUMINUM BURNED OUT NEAR EXHAUST VALVE SEAT. POSSIBLY CAUSED BY PRE-IGNITION OR DETONATION. APPROXIMATELY 300 HOURS PRIOR, ENGINE WAS CRACKING THE PORECELAIN IN THE SPARK PLUGS. MOST LIKELY HAPPENED DUE TO IMPROPER LEANING.