Autolite 25-4PK Copper Resistor Spark Plug, Pack of 4 Copper-glass seal bonds insulator, terminal post, and center electrode together to provide a tight seal that is 100 percent leak-proof Cold-formed steel shell and precision rolled threads, with one-piece terminal post to add strength Multi-rib insulator reduces danger of "flash-over" Resistor reduces radio frequency interference (RFI) and electrode erosion Full copper core electrode provides better performance in all conditions Mixture of nickel and copper-glass seal that bonds the insulator, terminal post and center electrode together, assuring full voltage Chromium-nickel alloy ground electrode resists gap erosion Copper-core center electrode sheathed in high chromium-nickel alloy for superior heat dissipation and durability Longer insulator nose provides a longer path to control fouling deposits