A certain amount of skill is required to position the gage properly. The user first moves the gage side to side until the indicator shows that the gage is on the centerline of the bore, and not measuring a chord of the circle. Then the gage is rocked up and down to set the contacts perpendicular to the bore axis. The minimum dimension shown on the indicator represents the bore's true diameter. Normally, the gage must be rocked back and forth several times, while the operator carefully watches the movement of the needle on the dial indicator.
Illustration shows how practice is needed to align the tool properly.
Incorrect alignment of bore gage to bore. The gage must still be rocked up and down. Some advanced digital indicators have a dynamic function that automatically captures the minimum reading, further simplifying the process. The user simply rocks the gage once, then reads the results. Provided they are manipulated carefully, rocking gages with centralizers are capable of high levels of accuracy.