Understanding hydraulic valve lifters

Understanding hydraulic valve lifters

By: Ray T. Bohacz: I have a love-hate relationship with valve-lash adjustment. I love adjusting anything mechanical: getting my hands on it and fine tuning it to perfection. The part I dislike about lash or free play adjustment is its awkwardness and complexity. Too many components have to be removed to perform a ten-minute task. For this reason, I like engines with hydraulic valve lifters that, for the most part, require no adjustment. However, when adjustment is necessary, instead of setting lash, as you would with solid or mechanical valve lifters, a hydraulic system requires preload as there is no lash. This is usually required only when the cylinder head is being reinstalled. The need for lash or free play The camshaft is responsible for the valve’s timing and its lift as well as its duration—the periods it remains open and closed. In a cam-in-block engine, this is accomplished by the camshaft working with intermediate components: valve lifter (or tappet), pushrod, and rocker arm. With an overhead cam design, the intermediate components differ, using some style of follower in lieu of a pushrod and possibly a tappet. This discussion focuses on the hydraulic tappet employed in cam-in-block engines. It’s the profile of the camshaft lobe that determines the valve action, and that motion is first transmitted to the valve lifter and onto the pushrod and finally the rocker arm that contacts the stem of the valve. When the parts are cold, they shrink and as heat is generated they expand. For this reason, free play is required to prevent parts binding when heated. Free play is created between the...