NGK 6431 BU8H Surface Gap Spark Plug. NGK&';s Surface Gap Plugs are Nearly Immune to Pre-Ignition, and Usually Require a Special High Energy Ignition System. They Can be Susceptible to Fouling, Especially in Colder Applications, but May be Necessary in High Compression Applications. Fits Mariner and Mercury outboard engines. Specifications: Shell Thread Size: 14mm Thread Pitch: 1.25mm Seat Type: Gasket Resistor: No Reach: 12.7mm (1/2") Hex Size: 13/16" (21mm) Terminal Type: Solid Overall Height: ISO Gap: .016" (0.4mm) Center Electrode Material: Nickel Type: Standard Size: 2.5mm Projection: Non-Projected Ground Electrode Material: Nickel Type: Surface Discharge Torque Specs: Cast Iron: 18-25.3 lb. ft. Aluminum: 18-21.6 lb. ft. Brand: NGK Heat Range: 8