As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
#4 TOP SPARK PLUG (TEMPEST) WAS SEIZED IN THE CYLINDER HEAD. SPARK PLUG APPEARS TO BE TORQUING AGAINST THE BASE OF THE HELICOIL RATHER THAN AT THE SPARK PLUG FLANGE AND THE GASKET. THE SPARK PLUG COULD NOT BE REMOVED WITH HEAT OR PENETRATING OIL, EVEN WITH MAXIMUM FORCE, AND SPARK PLUG SHEARED IN THE CYLINDER. SUSPECT MANUFACTURING DEFECT WITH THE CYLINDER SPARK PLUG HOLE. THE TAPPED SPARK PLUG HOLE ANGLE AND INSERTED HELICOIL APPEAR TO BE MISALIGNED.