Micrometer
A device used to measure extremely small dimensions, such as thickness or diameter, with high precision.
A micrometer (or micrometer caliper) is a precision measuring tool used by engine builders and technicians to measure very small distances, thicknesses, and diameters. It operates on the principle of a threaded screw, where rotating a thimble moves a spindle relative to an anvil. Micrometers are typically accurate to one ten-thousandth of an inch (0.0001 in) or one-thousandth of a millimeter (0.001 mm), making them far more precise than standard calipers.
Micrometers are essential for checking internal engine tolerances, such as crankshaft journal diameters, piston skirt clearances, valve stem diameters, and camshaft lobe wear. Using an inaccurate measurement in these areas can result in oil pressure loss or engine lockup due to lack of clearance for the oil film.
To ensure accurate readings, micrometers feature a ratchet thimble or friction sleeve that slips when a specific contact force is reached, preventing the user from over-tightening the tool and distorting the measurement. They must be calibrated regularly using a standard gauge block and stored in a protective case to prevent temperature-induced errors.