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What is tappet clearance, and why it is required ?

It is clearance between the valve stem and the rocker arm contact point.

This clearance is required to accommodate the thermal expansion of the running gear between the camshaft to the valve and to ensure positive closing of the valve at all working condition and temperature.

Often the requirement of tappet clearance is answered as, to maintain correct timing or change in clearance will lead to an early or delayed opening of the inlet or exhaust valves. Even though these are indirect effect of change in tappet clearance value and the one line answer should be to accommodate thermal expansion and ensure positive closing.

Excessive clearance leads to
1. Valve opens late
2. Closes early leading to decrement in scavenging efficiency.
3. Mechanical constraints like damage, noise and impact on the working surface.

Minimal clearance will lead to
1. Open early and closes late
2. Leaking of valve leading to burning of the valve seat and other constraints.
3. Compression pressure will also decrease due to leakage
4. Poor combustion and reduced engine power output.

Clearance should be monitored regularly using feller gauge for excessive or insufficient clearance

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