As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING NORMAL FLIGHT, PILOT NOTICED LEFT ENGINE RUNNING ROUGH AND RETURNED TO POINT OF DEPARTURE. DETERMINED THE NR 1 CYLINDER ON THE LEFT ENGINE HAD SEPARATED THE HEAD FROM THE BARREL. SUBMITTER SUGGESTED FROM PAST EXPERIENCE, THIS IS TYPICAL OF A TCM TSIO520 CYLINDER AND STATES THE ONLY COURSE OF ACTION THAT CAN BE TAKEN IS THOROUGH 50/100 HOUR INSPECTIONS.