, Notes 2: Trivalent metal plating provides superior anti-corrosion and anti-seizing properties, Notes 3: Corrugated ribs prevent flashover, Notes 4: Pure alumina silicate ceramic insulator, provides superior strength and better heat transfer, Notes 5: Copper core aids in heat removal, Notes 9: Fits Stihl Replaces Bosch USR7AC to fit 4 Stroke. Aluminum: 18-21. NGK Spark Plug, Individually Boxed NGK5246, Notes: NGK standard plugs are constructed for longer life and optimum performance.