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, 26.5mm (1.04") Reach, 5/8" (16mm) Hex Size, Gasket Seat, Resistor, Solid Terminal, Projected Electrodes, U-Groove Ground Electrodes, .044" (1.1mm) Gap