NGK Spark Plugs R5724-8: Spark Plug, Racing, Tapered Seat, 14mm Thread, .708 in. Reach, Projected Tip, Non-Resistor, Each - What's firing your engine? NGK racing spark plugs are built to withstand the severe mechanical and thermal shock associated with high-revving, high-compression engines. NGK racing plugs feature special electrode materials precious metals like platinum, iridium, and ruthenium for greater durability and low voltage requirements.