The drain plug size on Marine engines are determined by the make of the engines (I,e, Ford, Chevy, Volvo) used by the boat manufacturer(I.e. Mercruiser) Please find out the year and make of your engine, and use the Passenger Car/Truck engine application charts to identify the part number you need. If you are not sure, please contact your boat dealer to find out the thread size of the plug, and use the Valve Size Chart to determine the part number.