What is a 3 point micrometer

A 3-point micrometer (also called an internal micrometer or bore gauge) measures internal diameters using three symmetrically-spaced contact points that simultaneously touch the inner surface of a bore or hole. Unlike standard two-point measurements that can read across chords rather than true diameter, the three-point contact geometry automatically centers the instrument and measures the true diameter even if the micrometer isn't perfectly aligned with the hole axis.

This design incorporates three hardened anvils arranged 120 degrees apart on an expanding mechanism driven by a precision screw, similar to standard micrometers. As you rotate the thimble, all three contact points move radially outward simultaneously until they contact the bore wall. The geometry ensures that regardless of slight angular misalignment, the measurement reflects the actual bore diameter rather than a smaller chord measurement common with two-point instruments. Three-point micrometers are essential for precision bore measurement in cylinder bores, bearing housings, and machined holes where accuracy is critical and access allows only internal measurement. They're available in various size ranges (commonly 6-12 mm, 12-20 mm, 20-35 mm, etc.) since the expansion range of any single instrument is limited. For very deep bores, extension rods attach to the measuring head. While more expensive than standard calipers or two-point bore gauges, three-point micrometers provide superior accuracy for cylindrical hole measurement by eliminating the angular alignment issues inherent in simpler measurement methods.