For a flush fit every time, these gauges ensure your fastener and countersink mate perfectly for a secure hold. These gauges are often used to verify the top diameter and angle of holes in precision assemblies. They’re preferred over chamfer gauges, which also measure diameter but don’t check countersink angle. When the conical probe touches the hole’s angled walls, the diameter will display on the dial.

To verify the countersink angle, feel how snug the probe fits into the hole. You’ll also know the angle is incorrect when the reading for the top diameter is a different value than expected. Choose a gauge with an angle that matches that of your hole.

The dial on these gauges displays size deviations in real time. These gauges measure small slots and bores in precision parts, such as electronics, where even small size deviations can cause inefficiencies. Thanks to its full-ball design, the end of these gauges makes 360° contact inside the bore, so it's easier to center in tight spaces than anvil or spring-loaded contacts. This symmetrical ball is also less likely to tilt within the bore than half-ball gauges, which helps to avoid skewed measurements from improper placement.

Insert the probe into a bore and turn the knob on the handle to expand the contacts. To check size deviations, set the desired measurement with a ring gauge, bore gauge calibrator, micrometer, or caliper. The dial will display the magnitude of variation from the set measurement. To find the bore diameter, use a micrometer to measure the distance across the contacts.