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Denso U-Groove spark plugs feature a u-shaped groove in the ground electrode, giving the flame a groove to grow in, resulting in less flame dissipation and a larger flame front for a more complete combustion. This translates to better fuel efficiency and improved throttle response. Machine rolled threads prevent seizing or cross threading during installation and removal. 100% pre-fired to assure the highest quality possible. Purified alumina powder insulator provides exceptional strength and thermal conductivity. Five rib insulator prevents flashover. Copper-glass seal bonds center electrode and insulator for a gas-tight seal.