Hydraulic Lifter
A cylinder that uses oil pressure to automatically maintain zero valve clearance in the engine's valvetrain, reducing noise and wear.
A hydraulic lifter (also called a hydraulic lash adjuster) is a small cylinder positioned between the camshaft and the engine valves. Its primary job is to maintain zero clearance (or lash) in the valvetrain. It achieves this by utilizing pressurized engine oil that enters the lifter through a tiny check valve, expanding the lifter's internal piston to take up any slack between the cam lobe and the valve stem.
By keeping the components in constant contact, hydraulic lifters eliminate the need for manual valve adjustments, which were common on older solid-lifter engines. They also reduce wear on the valvetrain and ensure that the valves operate quietly and open to their full designed lift, maximizing engine breathing and performance.
Lifters can fail if their internal passages become clogged with oil sludge or if they leak pressure (a "collapsed lifter"). A bad hydraulic lifter is easily recognized by a loud, rapid ticking or tapping noise from the top of the engine, particularly when cold or at idle. Neglecting a noisy lifter can result in accelerated camshaft wear and reduced engine power. Swapping lifters requires removing the valve covers and camshafts.