Understanding hydraulic valve lifters

Understanding hydraulic valve lifters

By: Ray T. Bohacz: The most irritating aspect of valve-lash adjustment is its awkwardness; too many components require removal to perform a ten-minute task. Hydraulic valve lifters, on the other hand, require no adjustment for the most part. When adjustment is necessary, instead of setting lash, as you would with solid or mechanical valve lifters, a hydraulic system requires pre-load. 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, its lift and 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 rocker arm and the valve stem tip. Valvetrains that required lash were often identified as employing a solid lifter or mechanical camshaft. Today’s engines provide either a hydraulic or mechanical lifter, based on the manufacturer’s decision. Improvements in metallurgy and...