Metrology — Selecting the Right Gauge for Clearance To measure or check the small clearance (gap) between two mating surfaces, which type of gauge should be used?

Introduction / Context:
Precision assembly often requires verification of tiny gaps, such as valve clearances, shim stacks, or bearing end play. A simple, portable tool is standard for checking such clearances directly on the shop floor.

Given Data / Assumptions:

• The gap is narrow and planar between mating surfaces.
• Direct thickness comparison is acceptable.
• Accuracy required is on the order of hundredths of a millimetre (or thousandths of an inch).

Concept / Approach:
A feeler gauge consists of a set of precision-ground steel leaves of known thicknesses. By inserting single or stacked leaves into the gap to obtain a slight drag, the user reads the total thickness as the measured clearance. This is more practical than fixed-limit gauges (plug/ring/snap) for arbitrary small gaps and faster than setting up a micrometer in tight spaces.

Step-by-Step Solution:
Identify measurement target: a narrow gap between surfaces.Select an instrument that directly compares gap to calibrated leaf thickness.Use feeler leaves singly or in combination until a light drag is felt.Sum leaf thickness values to obtain the clearance.

Verification / Alternative check:
Automotive valve-lash procedures universally specify feeler gauges for setting and verifying clearances.

Why Other Options Are Wrong:
Ring/plug gauges are for checking hole/shaft sizes to limits, not gaps.Snap gauges verify external dimensions rapidly but not internal clearances between surfaces.A thickness micrometer can measure shims but is awkward to directly measure in-situ gaps without inserts; feeler gauges are purpose-built.

Common Pitfalls:
Using dirty or burred leaves leads to false readings; always clean and avoid forcing leaves which can distort them.

Final Answer:
feeler gauge