Abstract: A hydraulic lash adjuster mechanism for an internal combustion engine, the adjuster having a body having a bore formed therein with a piston slidingly received within the bore. The lash adjuster is in communication with one end of a cam follower that is in communication at its other end with a valve stem. A rotating cam contacts said cam follower to apply force to the piston during a valve lift event. The lash adjuster has a low pressure chamber formed in the piston and a high pressure chamber formed between the bottom of the bore and the bottom of the piston. Engine fluid that is passed to the low pressure chamber is in communication with the high pressure chamber through a valve opening. A mechanism selectively opens or closes the valve opening in response to pressure differences between the low pressure chamber and the high pressure chamber.