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Valve Lash

What is Valve Lash?


Valve Lash
Valve Lash - Image courtesy of diamondbackengines.com
Valve Lash or Valve Clearance is the gap between the Rocker Arms and the valve tappet. This clearance must be tightly controlled. If it is too little, the valves may not seat properly. If it is too much, you create valve train noise, and excess load on the valves and valve train components.

As the picture shows below, valve lash basically cuts off the lower portion of the cam profile. This lower section is where the cam profile is designed to gently open and close the valve. If you have too much valve lash, you eliminate the more "gentle" sections and are basically "slamming" the valve open and closed.


Too Much Valve Lash
Too Much Valve Lash
Because of manufacturing tolerances, engine wear, and because different parts of the valve train and engine "grow" as the engine heats up, this clearance can change and must be set correctly. Camshafts are ground assuming a certain amount of valve lash. If you change it, you risk changing the performance and durability of what the cam grinder designed into their lobes.

Over Head Cams

On overhead cam engines the pushrods are eliminated and sometimes the rocker is not included so the valve is operated directly from the Cam Lobe. But there is still valve lash. Over Head cam engines are presumed to be higher performance engines and usually have dual over head cams for 4 or 5 valve engines but some experts argue that they are more complex and pushrod engines are simpler, cheaper, more compact, and have less Engine Friction.

Hydraulic Lifters


Valve Lash
Valve Lash - Image courtesy of buyspeedparts.com
Hydraulic Lifters were introduced in the 1950's as a way to make mass produced engines quieter, and not require the maintenance of periodic valve lash adjustment. They use oil pressure to pump up a "plunger" when the valve is on it's seat to eliminate any clearance, basically eliminate valve lash. This plunger has a small amount of leakage, so oil can slowly leak out so the plunger does not hold the valve off it's seat when the valve should be closed. But in the short amount of time that the valve is open, very little oil can leak out so it acts as a solid lifter then. Actually, oil does leak out when the valve is open, and the oil is NOT incompressible. For these reasons, a hydraulic lifter is assumed to have about .006" of effective lash.

Hydraulics are typically not used on full race motors. At higher RPM they can "pump up", meaning there is so little time for bleed down, they start to hold the valve off it's seat. This can cause a loss in performance and can drastically cause valve temps to go up (especially the exhaust) because the valves are cooled by transferring heat to the seat. Hydraulics are typically heavier and "springier", so they add additional problems to keeping the valve train under control at high RPM.

Programs that take Valve Lash into account

We have 3 programs that work with Valve Lash when conducting Engine SimulationEngine Analyzer v3.4, Engine Analyzer Plus v3.4. And Engine Analyzer Pro v3.9 also uses Valve Lash in Engine Simulation.

And we have 5 other programs that take Valve Lash into account when making calculations. Cam Analyzer v3.8, Valve Spring Tester v1.1, Port Flow Analyzer v3.5 with Valve Opener and Engine Log Book v1.1.

Valve Lash
Valve Lash - Image courtesy of cumminsforum.com
Valve Lash
Valve Lash - Image courtesy of satikudlu.blogspot.com
Valve Lash
Valve Lash - Image courtesy of arrc.epnet.com