What was found
PILOT REPORTED THAT AN ENGINE FAILURE OCCURRED DURING FLIGHT WITH UNEVENTFUL LANDING. PILOT INFORMED TECHS THT THE TOP SPARK PLUG HAD BECOME REMOVED FROM THE NR 1 CYLINDER. UPON ARRIVAL TO THE AIRCRAFT, FOUND TOP COWLINGS REMOVED AND THE NR 1 TOP SPARK PLUG LAYING ADJACENT TO THE CORRESPONDING CYLINDER, SPARK PLUG BOSS. THE NR 1, TOP SPARK PLUG BOSS HELICOIL WAS STILL INSTALLED IN THE CYLINDER. REPLACED THE EFFECTED HELICOIL WITH NEW AND SECURED HELICOIL IN PLACE, USING CARE NOT TO CONTAMINATE THE CYLINDER. NOTED THE CYLINDER BOSS THREADS HAD TO BE TAPPED OR CLEANED TO ACCEPT THE NEW HELICOIL. CYLINDER BOSS APPEARED TO NOT BE FULLY TAPPED AND BOSS THREADS MAY HAVE BEEN TAPERED.