Spark plugs are an essential part of a motor. The function of a spark plug is two part. The first is to ignite the air/fuel mixture in the combustion chamber. The second part is to remove heat from the combustion chamber and deliver it to the cooling system of the engine. Different engines -- whether in a car/land or boat/marine vehicle -- have different needs. The spark plug must remain cool to prevent a second ignition while delivering the heat to its next destination.
A marine spark plug with cold-rolled threads prevents the plug from seizing or not sparking. This means the spark plug will still ignite even if it is installed in an aluminum chamber. Marine spark plugs are also able to resist salt water corrosion with cold-rolled threads better than spark plugs made for a car without these special threads.
Another difference between marine spark plugs and those in a car is that marine spark plugs have a double-dipped coating of zinc chromate-coated metal. This shell keeps the spark plug from corroding as well. Should a spark plug corrode, it does not cool down as needed and may re-ignite, which can cause a mis-fire or, worse still, the engine to catch fire.
A boat motor may work mainly in a slow trolling environment, or it could be called upon to perform at full speed for long periods of time. The spark plug must be able to handle the amount of heat the engine will generate. A marine spark plug has a large heat range, depending on its size. Too small of a range and the driver risks short-circuiting the engine and causing a misfire.
According to "Marine Engine Digest," the above differences are the only ones that distinguish a marine spark plug from an automotive plug. Use the spark plug recommended in the owner's manual of your vehicle for how much heat it can handle and re-distribute. Maintaining your vehicle, by following the standards recommended in the owner's manual, will ensure smooth running of your boat or car engine.
