Automobile engineering — measurement tools Which instrument is specifically used to measure valve tappet (valve lash) clearance during engine maintenance?

Introduction / Context:
Correct valve tappet clearance (also called valve lash) is critical in internal combustion engines. Too much clearance causes noise and reduced valve lift; too little clearance can lead to valves not seating fully, overheating, and loss of compression. Measuring this small gap precisely is a routine yet vital servicing task.

Given Data / Assumptions:

• The gap to be measured is very small, typically in the range of 0.05 mm to 0.40 mm depending on engine and whether the valve is intake or exhaust.
• Measurement is taken with the cam lobe on its base circle (valve fully closed) or at the specified crank angle.
• Ambient workshop conditions; no special electronics assumed.

Concept / Approach:
The industry-standard tool for gauging small, known clearances is the feeler gauge, a set of calibrated steel blades of known thicknesses. The correct blade should slide with a slight drag between the rocker/adjuster and the valve stem tip (or cam and bucket/shim in overhead cam designs). Other listed instruments are not intended for such tiny, in-situ gap checks.

Step-by-Step Solution:
Ensure the engine is at the specified temperature (often cold) and the valve is fully closed.Select a feeler gauge blade matching the manufacturer’s specified clearance.Insert the blade between the contact faces; feel for light resistance (“go”) and check that the next thicker blade does not enter (“no-go”).Adjust using the locknut/screw (if applicable) until the specified feel is achieved; recheck after tightening.

Verification / Alternative check:
Confirm by trying one size thinner (should be easy) and one size thicker (should not enter). Record values for all valves.

Why Other Options Are Wrong:
Screw pitch gauge measures thread pitch, not clearance.

Engineering scale cannot resolve such small gaps reliably.

Vernier caliper and micrometer measure external/internal dimensions, not an in-assembly clearance with correct tactile feedback.

Common Pitfalls:
Measuring on the cam lobe instead of base circle; mixing metric and inch blades; not accounting for hot lash specifications where required; over-tightening the locknut and changing the setting.

Final Answer:
feeler gauge