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Dial-bore gauges are comparators, which means you must use a dedicated calibration ring, a gauge fixture, or a micrometer to set the diameter you wish to measure. We always use the same micrometer for both measuring journal diameter and setting up the dial-bore gauge. For this photo, we’re using a 2- to 3-inch micrometer set to 2.20 inches. Note the short extension plugged into the dial-bore adapter (arrow). We used this same procedure to set up our bore diameter of 3.790, which approximated the bore diameter of the cylinder we were measuring.