Inside mikes are used to measure bore sizes. They are frequently used with outside mikes to reduce the chance of error. To use an inside mike, place it inside the bore or hole and extend the measuring surfaces until each end touches the bore surface. If the bore is large, it might be necessary to use an extension rod to increase the measuring range. Extension rods come in various lengths. The inside micrometer is read in the same manner as an outside micrometer.
To obtain a precise measurement in either inch or metric graduations, hold the anvil firmly against one side of the bore and rock the inside mike back and forth and side to side. This ensures that the mike fits in the center of the work with the correct amount of resistance. As with the outside micrometer, this procedure will require a little practice until you get the feel for the correct resistance and fit of the mike. After taking a measurement with the inside mike, use an outside mike to take a comparison measurement. This reduces the chance of errors and helps ensure an accurate measurement.