Denso Spark Plug Denso s standard plugs have always offered a patented U-groove ground electrode producing for a larger hotter spark. Copper core for better thermal conductivity. Machine rolled threads to reduce cross-threading and seizing . The plug insulator is constructed of purified alumina powder for extreme stress with 5 ribs to prevent flashover. Technical Specifications - 14mm Thread 19mm (3/4 ) Reach 5/8 (16mm) Hex Size Gasket Seat Resistor Solid Terminal Projected Tip U-Groove Ground Electrode .044 (1.1mm) Gap