NGK Iridium IX Spark Plugs TR 55 1 IX NGK's IX Iridium Series Plugs were Designed Specifically for the Performance Enthusiast. Fine Wire Center Electrode Ensures High Durability and a Consistently Stable Spark Iridium Alloy has an Extremely High Melting Point, Perfect for Today's High-Tech, High Performance Engines Trivalent Metal Plating Provides Superior Anti-Corrosion and Anti-Seizing Properties Corrugated Ribs Prevent Flashover Pure Alumina Silicate Ceramic Insulator, Provides Superior Strength and Better Heat Transfer Copper Core Aids in Heat Removal Triple Seals Prevent Leakage Plug Specifications Thread Size 14mm Hex Size 5/8" (16mm) Reach 17.55mm (.691") Seat Type Tapered Gap .040" (1.0mm) Heat Range 6 Terminal Type Solid Resistor Yes Thread Pitch 1.25mm Brand NGK Note Picture is generic and may not represent actual product.