Measure the diameter of small holes, grooves, and recesses. The end of these gauges has a 360° contact surface that’s easier to center in tight spaces than anvils or spring-loaded contacts. Use them to measure slots and bores in precision parts, such as electronics, where small size deviations can cause inefficiencies.

Insert the probe into a bore and turn the knob on the handle to expand the contacts. To find the bore diameter, use a micrometer to measure the distance across the contacts.

Half-ball gauges are best for measuring near the bottom of flat, shallow bores.