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Adjusting Valves

This primer is rooted more in understanding the need to maintain valve lash on all engines instead of reviewing the procedure. Steps required and the clearance will be application-specific to the engine.

The common objection to doing valve adjustment is the ordeal of getting to the service site.

After reading this, I trust you will find the chore worth the effort.

Contrary to popular belief, the cylinder bore is not filled to capacity with charge (fuel/air mix) on a normally aspirated (NA) engine. The amount of cylinder fill is measured in a percent and is called volumetric efficiency (VE). An NA engine experiences approximately 80% VE, and that occurs at peak torque.

The purpose of a turbocharger is to increase the VE to 100% or more. Depending on boost and airflow capability, VE can go beyond that.

The intake valve is the gateway to the cylinder bore. If the valve lash is excessive, then valve lift is lost. In turn, intake port flow drops along with power and torque.

Regarding the exhaust valve, decreased lift limits cylinder evacuation during the exhaust stroke and costs power and longevity due to excessive heat.

Wrong valve lash also impacts the valve timing and engine reliability.

If the valve lash clearance is too tight, then the valve will open earlier and close later than intended. If it is too loose, the valve will open later and close earlier.

The effect of setting the valve lash too tight can result in the valve not fully closing on the seat.

This will allow cylinder pressure to escape along with high-temperature combustion gases. It is possible then for the temperature of the valve head and valve seat to become so high that rapid burning can occur. This has the potential to flame-cut the cylinder head beyond the seat and valve face. This is usually further aggravated by charring of the oil film on the valve stem, which causes the valve to stick in the guide.

If the valve lash is excessive from a lack of adjustment, the engine is noisy and VE suffers.

Excessive wear results from the valve pounding against its seat. 

Beyond adjustment
Adjusting the valves also affords the opportunity to look things over and to spot potential problems.

The state of the lash is also telling. A valve that is found to be too tight before adjustment usually is indicative of either valve seat recession or the valve pulling through the cylinder head.

Either situation is a serious problem if left unaddressed.

In contrast, excessive clearance can reveal a valve stem that is mushrooming or can show worn parts such as the rocker or adjustment mechanism.

You will find differences on each cylinder, but they should be minimal.

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