4 New NGK Iridium IX Spark Plugs TR7IX # 3690, Iridium IX Plug, 14mm Thread Size, 17.55mm (.691") Reach, 5/8" (16mm) Hex Size, Tapered Seat, Resistor, Solid Terminal Nut, Projected Tip, Fine Wire (0.6mm) Iridium Center Electrode, Tapered Cut Ground Electrode, .040" (1.016mm) Gap, Heat Range 7