Small hole gauge sets include indirect measurement tools that are used to determine the inside diameter of small bores and grooves, up to 0.5 inches. The two-point contact style consists of two half-ball hardened contacts that expand from closed until they make contact with the sides of the bore. Operators turn a knurled knob or nut at the head of the gauge to expand or contract the contacts. When contact has been made, the user extracts the gauge from the hole and uses an outside micrometer to measure the gauge's setting, which indirectly gives the diameter of the hole.