Valve Lifter (Tappet)

The valve lifter is the unit that makes contact with the valve stem and the camshaft. It rides on the camshaft. When the cam lobes push it upwards, it opens the valve. The engine oil comes into the lifter body under pressure. It passes through a little opening at the bottom of an inner piston to a cavity underneath the piston. The oil forces the piston upward until it contacts the push rod. When the cam raises the valve lifter, the pressure is placed on the inner piston which tries to push the oil back through the little opening. It can't do this, because the opening is sealed by a small check valve. When the cam goes upward, the lifter solidifies and lifts the valve. Then, when the cam goes down, the lifter is pushed down by the push rod. It adjusts automatically to remove clearances.