Inside micrometers and bore gauges are precision measuring instruments used to measure or transfer dimensions on a workpiece. Inside micrometers measure interior distances on a workpiece, such as the span between two edges, or the diameter of a circle or hole. Bore gauges measure the interior diameter of holes, or bores. While a bore gauge can measure the interior diameter of a hole at any depth, a caliper-type inside micrometer can only measure to the depth of its jaws. Rotations of the instrument’s calibrated screw amplify small distances that cannot otherwise be measured directly, translating those turns to a measurement that can then be read from a scale, dial, or electronic display.

A caliper-type inside micrometer has a pair of jaws that can be inserted into the space to be measured from interior edge to edge. Tubular and rod inside micrometers are placed within the space to be measured, and are extended to fill the remaining span and measure the interior wall to wall.

Bore gauges have a two- or three-point contact, which indicates how many points on the gauge head touch the inside of the bore. Two-point contacts are better at measuring ovality, while three-point systems are used to measure lobing, or triangular form error. Pistol-grip bore gauges are used for fast, single-handed operation with good repeatability from one operator to another. Bore gauges, regardless of type, can be used in some special applications, such as measuring threads, spline pitch diameters, and deep bores.

Inside micrometers and bore gauges are commonly made of steel. A micrometer’s measuring face may have carbide tips for additional hardness and to help extend tool life. Similarly, the contact points of a bore gauge may be made of carbide.

When selecting the appropriate inside micrometer or bore gauge for an application, consider the material type, system of measurement, and maximum measurement.