Measure a variety of hole diameters. These sets include gauges that use spring tension to expand the probe’s contacts inside of bores. Each gauge has a different measuring range, so they’re useful for general checks. Use them to test engine cylinders, machine components, and turbines for deviations in roundness that could lead to wear or inefficiency. 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.