As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ACFT STARTED AND RUNNING FOR A TRAINING FLIGHT, PILOTS NOTICED A VIBRATION OF THE ENGINE AND NO CYLINDER HEAD TEMP OR EXHAUST TEMP ON NR 6 CYLINDER IAW THE ENGINE MONITOR. PILOTS SHUT ENGINE DOWN AND NOTIFIED MAINTENANCE. AFTER INSPECTION OF CYLINDER NR 6, BY USE OF A BORSCOPE FOUND EXHAUST VALVE WAS MISSING A PORTION OF THE VALVE. REMOVED THE CYLINDER AND FOUND IT CRACKED FROM THE LOWER SPARK PLUG BOSS TO THE EXHAUST PORT, DAMAGE TO INNER CYLINDER HEAD MISSING MATERIAL AND MISSING PORTION OF EXHAUST VALVE.