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Before measuring the bore, you must use the provided tool (arrow) to remove the thread-in extension and install the proper one for the bore size you intend to measure-the included instructions tell you which extension corresponds to what bore size. However, you still need to use an outside mic to precisely set the size of the measuring end of the dial-bore gauge. We like to preload and zero the gauge at a size slightly larger than that to be measured, then subtract that amount from the indicated bore measurement in order to find the true bore size. With this method you can check both bore oversizing as well as undersizing. For example, if the dial gauge anvils are set at 2 inches and cannot travel any larger, then it would be impossible to measure any oversizing of a bore that is spec'd at 2 inches.