Denso Spark Plug Multi-ground electrodes specified by engine manufacurer for unusual combustion chamber design, click on Multi-ground for more info. 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, Removable Terminal Nut, Dual Ground Electrodes