tappet

A tappet, in automotive terminology, is a component of an internal combustion engine’s valve train that acts as the intermediary between the camshaft and the valve. It translates the cam lobe’s rotary motion into linear movement that opens and closes the intake and exhaust valves, typically via pushrods and rocker arms in OHV designs or directly in some OHC configurations.

There are several common types. Flat tappets have a flat contact surface that rides on the cam lobe and are common in older engines; they require proper lubrication and can be susceptible to wear if the cam and lifter surfaces are not in good condition. Roller tappets incorporate a small roller bearing to reduce friction and wear between the lifter and cam lobe. Hydraulic tappets, or hydraulic lifters, contain a fluid-filled chamber and a plunger that auto-adjusts valve clearance to minimize lash and noise. Solid (or mechanical) tappets have no hydraulic adjustment and require manual lash setting, often used in high-performance applications. Bucket tappets, used in some OHC engines, sit on the cam lobe and transfer motion to a rocker arm, typically with a shim to set clearance.

Operation and maintenance rely on proper lubrication with engine oil, critical for reducing wear at the contact surfaces between cam lobes and tappets. Wear, incorrect break-in, or oil contamination can cause tappet scuffing, reduced valve timing accuracy, or noisy operation. Tappets are typically made from hardened steel alloys and are designed to withstand high contact stresses in the valve train.