Just like a car engine, a boat’s motor is a combustion engine that uses fuel and spark plugs to drive pistons that—in turn—rotate the propeller that drives a boat through the water. In a perfect world, engine parts would last forever. But constant friction means these parts eventually need service and replacement. One of the most common parts for replacement is a boat’s spark plugs. Spark plugs for boats do one simple job: when the piston is driven up toward the plug, a small amount of fuel is introduced into the area between them. The spark plug ignites the fuel, which creates small, contained combustion that drives the piston back down. Eventually, though, spark plugs simply wear out. They don’t fire as strongly, which means the boat engine doesn’t run as smoothly or may have difficulty starting. When this happens, it’s time for a boat engine tune up. Boat spark plug tune up kits include the number of spark plugs necessary for a specific boat engine, and usually spark plug wires as well. Because plug wires carry current and also wear down—which can lead to leaking electricity that arcs to other metal parts of the engine—it’s usually recommended to change both the boat spark plugs and the plug wires at the same time. Use our Marine Engine Parts Selector Tool to find the right engine parts for your boat.