What was found
A SPARK PLUG WAS REPLACED DURING A SCHEDULED MAINTENANCE INSPECTION. DURING THE POST-MAINTENANCE OPERATIONAL CHECK, THE ENGINE SUDDENLY DEVELOPED ROUGHNESS AND A PARTIAL LOSS OF POWER. SUBSEQUENT INSPECTION FOUND THE SPARK PLUG HAD COMPLETELY SEPARATED INTO TWO PIECES AT A POINT BETWEEN THE SPARK PLUG GASKET BOSS AND THE HEXAGONAL NUT PORTION (WHAT TEMPEST REFERS TO AS THE HOT LOCK ASSEMBLY). THIS CAUSED A LOSS OF COMPRESSION ON THAT CYLINDER AND IGNITION FAILURE OF THAT SPARK PLUG. A PRECAUTIONARY CHECK OF SPARK PLUGS IN HELD IN STOCK DISCOVERED FOUR SPARK PLUGS WITH CRACKING IN THIS AREA (STILL SEALED IN SEALED FACTORY PLASTIC SLEEVES. THE SPARK PLUGS WERE FROM LOT # 1VGM.