Inspect a variety of hole sizes with the gauges in these sets. Each has spring-loaded contacts on the probe, which expand inside holes to measure diameter. These gauges also detect deviations in roundness, so they can help you make sure parts don’t wear prematurely or run inefficiently. Use them for general checks when testing engine cylinders, machine components, and turbines. Designed to reach down into bores, they measure the diameter of deep holes better than calipers and inside-hole micrometers.

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.