Nickel Plugs feature two flat surfaces, which block the ability of the ignition spark to grow. NICKEL TWIN TIP SPARK PLUGS. Life span of 120,000 km. DENSO’s cutting-edge technology has created a unique iridium rhodium alloy with the highest iridium ratio in the market for the center electrode as well as a bespoke platinum alloy for the ground electrode, allowing for the extended lifespan of the plug.