Dial bore gauges are typically used by machinists to measure the inner diameter of bores in motor engines. Tresna Instruments says the cost associated with bore gauges means they are often found only in the hands of professionals. If you can afford one, the speed and accuracy of a dial bore gauge can save a lot of time when used to measure cuts or hole sizes in home remodelling projects. Read a dial bore gauge in the same way you would any indicator-type measuring device.
Place the bore gauge in a micrometer locked to the same diameter of the hole being measured.
Rotate the indicator's dial until the pointer aligns with the "0" marking on the dial face.
Insert the dial bore gauge in the bore, ensuring both anvils of the gauge touch the wall. Gently rock the bore gauge back and forth while keeping the anvils along the same axis plane. The pointer will swing either clockwise or counter-clockwise as the anvils settle into place square to the bore.
Watch the dial face once the tool is squared. At some point the pointer will start to reverse itself and head in the opposite direction. This could be on either side of the "0" marking on the indicator. Record the measurement where the pointer reverses itself and remove the bore gauge from the hole.
Add or subtract the recorded value from the target number the indicator was calibrated to. If the bore gauge's indicator was zeroed to 2 inches, and the pointer changed direction at the .007-inch marking on the dial face, the final measurement of the hole is 2.007 inches. Alternatively, if the pointer travelled counter-clockwise and changed direction .007 inches to the left of the "0" on the indicator, the final measurement of the hole is 1.993 inches.