Definition
Thin metal blades, each precisely manufactured to a specific thickness, used to measure the size of small gaps between two parts. The blades are marked with their thickness (for example, 0.005 inch) and are selected individually or stacked together until they just fit the gap with light drag, indicating the gap's measurement.
Plain English
A set of thin metal strips of known thicknesses used to measure how wide a small gap is. You slide a strip into the gap, and when it fits with a slight friction, the number stamped on that strip tells you the gap size.
Context Anchor
Seen in aircraft maintenance tasks where a mechanic must check or adjust small gaps in engine or airframe parts.
Derivation
The word 'feeler' is used because the technician 'feels' the fit of the blade in the gap rather than reading a number off a dial. The slight drag as the blade slides through is the feedback. 'Gage' is an older spelling of 'gauge,' meaning a measuring tool.
Why Pilots Care
Many engine clearances are critical to safe operation. A spark plug gap that is too wide or too narrow affects ignition; a valve clearance out of spec affects engine timing and cooling. Feeler gages are the standard tool for verifying these clearances are within manufacturer limits.
Intuition Check
Do not picture a round cockpit gauge. Feeler gages are thin measuring blades; the marked thickness on the blade tells the size of the gap when it fits correctly.
Example Sentence 1
The mechanic used feeler gages to check that each spark plug gap matched the value specified in the engine manual.
Example Sentence 2
After torquing the cylinder, she rechecked the gap with several feeler gages to confirm the measurement remained within limits.