Mercury - ACDelco GM Original Equipment 41-983 Double Platinum Spark Plug MerCruiser Big Block Ignition Series Double Platinum Finish 8M0129146. Overview. There's a special kind of frustration when a big sterndrive engine doesn't feel crisp. It might still run, but it doesn't feel clean. Starts take a little longer, idle gets a touch rough, and you catch yourself listening harder than you should. Spark plugs are small parts with a big voice, and when they're fresh and correct for the engine, you