A bore gauge measures the inside diameter of holes with precision down to a ten-thousandth of an inch. Commonly used during engine rebuilds, it checks if cylinder bores are worn, tapered, or out-of-round. The tool consists of a long shaft with a dial indicator on top and a measuring head with spring-loaded contact pins on the bottom. You calibrate the gauge using a micrometer, insert it into the bore, and rock it back and forth to find the minimum reading.
Cylinders do not wear evenly. They tend to wear wider at the top where combustion pressure is highest, and out-of-round due to the side thrust of the pistons. The bore gauge tracks this wear at different depths, helping you decide if you must bore the cylinders larger or if they can be honed.