Definition
A precision measuring instrument used to measure the inside diameter of a hole, bore, or cylinder to within a thousandth of an inch (0.001") or finer. It consists of a calibrated thimble and spindle that extend outward against the inner walls of the part being measured, with the reading taken from the scale on the barrel and thimble.
Plain English
A precise tool for measuring the width of a hole or the inside of a cylinder. You place it inside the hole and turn it until it just touches both walls, then read the size off the scale.
Context Anchor
Seen in aircraft maintenance when checking internal wear or fit, such as inside a cylinder, sleeve, tube, or machined hole.
Derivation
Micrometer comes from the Greek 'mikros' (small) and 'metron' (measure) -- literally 'small measure.' 'Inside' distinguishes it from an outside micrometer, which measures the external dimensions of a part.
Why Pilots Care
Verifies critical engine tolerances such as cylinder wear and piston clearance to maintain airworthiness and prevent failures.
Intuition Check
Do not confuse an inside micrometer with a tool for measuring outside thickness. If the measuring faces touch two inner walls of an opening, it is being used as an inside micrometer.
Example Sentence 1
The mechanic used an inside micrometer to check the cylinder bore for wear before deciding whether the engine needed to be overhauled.
Example Sentence 2
Readings from the inside micrometer confirmed the piston-to-wall clearance met specifications.