Spring-loaded contacts on the end of the probe expand inside holes, making these bore gauges versatile enough for general checks. They’re often used to inspect vehicle parts, such as engine cylinders, for deviations in roundness that could lead to wear and inefficiency. They’re also useful for measuring a deep hole’s diameter. Compared to calipers and inside-hole micrometers, they reach deeper into holes to take measurements.

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.