Take good care of the cooling system - change to a new impeller every year. The impeller is very important. Its task is to pump water through the cooling system of the engine. Remember the impeller should be replaced every year and always make sure you get a Johnson original impeller, then you'll know it matches your pump exactly. Made of oil-resistant nitrile. Outer Diameter: 2.56"/Inner Diameter: Splines/Width: 1.98"/Replaces Jabsco: 17937-0003.