Versatile enough for general checks, these bore gauges have spring-loaded contacts on the end of their probe to expand inside holes. They’re often used in vehicle repairs and to test parts, such as engine cylinders, for deviations in roundness that could lead to wear. Since they reach father into holes than calipers and inside-hole micrometers, these gauges make it easier to measure a deep hole’s diameter.

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.