Valve train maintenance: Why is it necessary to maintain specified valve clearances (lash) in an overhead-valve or overhead-cam engine?

Introduction / Context:

Valve lash is the small clearance between cam-operated components (cam and tappet, rocker tip and valve stem) when the valve is fully closed and the cam is on its base circle. Correct lash is vital for sealing, durability, and precise timing. This question asks for the primary reason engines specify and adjust valve clearances.

Given Data / Assumptions:

• Mechanical (solid) lifter systems or adjustable rocker arms; hydraulic tappets auto-compensate but still rely on design lash concepts.
• Hot running conditions elongate valves and change geometry.
• Clearance measured cold unless specified otherwise by the OEM.

Concept / Approach:

Valves, particularly exhaust valves, expand (lengthen) as temperature rises. Lash provides room for this thermal growth so that, at operating temperature, the valve still seats fully for compression sealing and heat transfer to the seat. Insufficient or zero lash can hold a valve slightly open when hot, causing loss of compression, burning, and rapid failure. Excessive lash leads to noisy operation, impact loading, and altered valve timing (reduced effective duration/lift).

Step-by-Step Solution:

Verification / Alternative check:

Service manuals specify different cold/hot lash values; exhaust lash is often slightly larger than intake due to higher temperatures, confirming the role of thermal expansion.

Why Other Options Are Wrong:

• Reduce sliding resistance: lubrication, not lash, primarily controls sliding friction.
• Increase valve speed: excessive lash impairs timing and durability.
• Make crankshaft smoother: lash does not directly smooth crank rotation.
• Hydraulic lifters only: hydraulics self-adjust; the principle applies generally to account for expansion.

Common Pitfalls:

• Setting lash too tight after a valve grind; hot running then holds the valve off its seat.
• Ignoring OEM specified hot vs cold measurement conditions.

Final Answer:

allow for lengthening (thermal expansion) of the valves due to combustion heat