Autolite 306 Copper Resistor Spark Plug, Pack of 1 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