A different measuring range on each gauge makes these sets handy for checking a variety of hole sizes. Each gauge has spring-loaded contacts on the probe, which expand inside holes to measure diameter. Often used for general checks, these sets can make sure that engine cylinders, machine components, and turbines don’t deviate in roundness, which could lead to wear and inefficiency if undetected. Since the gauges reach into holes farther than calipers and inside-hole micrometers, they make it easier to inspect deep holes.

Once the contacts expand inside the hole, turn the screw on the handle to lock the contacts in place. To find the bore diameter, use a micrometer to measure the distance across the contacts.