Mercury - Quicksilver Spark Plug Ignition System Component NGK BU8H Semi-Surface Gap 13686Q. Overview. A smooth, clean spark is the heartbeat of your engine, and the Mercury Marine 13686Q Quicksilver Spark Plug (NGK BU8H) keeps that heartbeat steady. Built with a semi-surface discharge design, it delivers efficient ignition and helps burn away carbon buildup before it can foul your plugs. Whether you're easing out of the marina or opening up on the bay, this plug keeps your outboard firing crisp