2001- Later GMT800, 2500hd, 3500 with 6.6 liter Duramax diesel engine GM-44640 is used to install valve stem seals to the correct depth. Prevention of oil leaks into the cylinder is critical to the operation of the engine and to the environment. GM-44640 is used with or without the valve in place. Position GM-44640 over the valve stem seal. Apply a downward force until the seal is seated into position.