What Is a Valve Lash?by Pauline Gill
Valve lash is the mechanical clearance in the valve train between camshaft and valve in an internal combustion engine. Valve lash is usually about 10/1,000 of an inch depending on the engine specifications. Valve lash is intended to provide the greatest amount of valve opening on the high point of the camshaft lobe and assure that the valve is tightly closed on the low segment of the camshaft lobe. Valve lash adjustment is necessary maintenance for engines without hydraulic valve lifters.
One of the most complex issues intrinsic to internal combustion engines is that their characteristics change depending on temperature. Because different metals and different metal shapes expand at different rates, the clearances between moving metal parts may vary widely as the engine approaches its optimum operating temperature. Most engines are designed to operate efficiently and quietly, not bind, and have a long life running at this temperature. All of these considerations are taken into account when determining a valve lash setting, as the clearance varies at different engine temperatures.
Hydraulic valve lifters started to become predominant in the fifties as the car-buying public demanded more smoothness and refinement in their auto engines. Hydraulic valve lifters use engine oil pressure to establish a continuous zero valve lash dimension under all conditions in the auto engine. While the valve is closed, the internal piston of the lifter is lightly thrust against the pushrod by engine oil pressure to eliminate all valve train clearance. When the camshaft high spot comes around, the hydraulic lifter's fill hole is covered and the lifter acts like a solid piece of metal, and the valve opens. Hydraulic lifters become noisy when they get plugged.
Hydraulic lifters add mass and complexity to engines that complicate the quest for high performance. Therefore, most performance builders use solid lifters, stiffer valve springs, and sturdier valve train parts like pushrods and rocker arms. These engines have greater tolerances in general, which is why they are much noisier than docile engines.
Overhead camshafts eliminate much of the overhead valve drive train and with it most of the variation in the systems mechanical clearances.
Non-hydraulic overhead valve systems require re-adjustment as engines wear and change their dimensional characteristics. Most manufacturers recommended adjusting valve lash with the engine warmed up, and running, if at all possible, since again, temperature is the biggest single issue with valve lash.
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