A dial bore gauge measures the inside diameter of engine cylinder bores. It uses a long shaft with a dial indicator at the top and a measuring head with pivoting contacts at the bottom. By inserting the head into the cylinder and rocking the gauge, you find the minimum reading on the dial face. This gives you the exact measurement down to ten-thousandths of an inch. Engine builders rely on this tool to check for cylinder wear, taper, and out-of-round conditions before ordering pistons.
To get accurate readings, you must calibrate the gauge first. You set it using a micrometer or a master setting ring at the engine block's standard bore size. Take measurements at the top, middle, and bottom of the cylinder. Measure both parallel and perpendicular to the engine block. The differences between these numbers reveal if the cylinder has worn into an oval shape or narrowed at the bottom.