4 New NGK IRIDIUM IX Spark Plugs TR55-1IX # 7316, Iridium IX Plug, 14mm Thread Size, 17.55mm (.691") Reach, 5/8" (16mm) Hex Size, Tapered Seat, Resistor, Extended Projected Tip, Solid Terminal Nut, Extended Metal Shell, Fine Wire (0.6mm) Iridium Center Electrode, Tapered Cut Ground Electrode, .060" (1.52mm) Gap, Heat Range 5NGK Spark Plug