A bore gauge is a tool used to measure the inside of a bore, or hole. Once a bore gauge is inserted into the hole that needs measuring, small parts called anvils expand outward to determine the diameter. Bore gauges are also known as cylinder tests, hole tests, bore mics, holtests, internal micrometers, hold bore gauges, or telescoping gauges. There are several types of bore gauges. Bore gauges with three anvils are called internal micrometers or tri mics, and are calibrated with setting rings. The more common, and less expensive, type of gauges feature two anvils and are calibrated with gauge blocks. Plug gauges are the simplest type; they feature a plug of slightly different size on each end. A correctly sized bore will not be able to fit the larger plug inside. Both three and two anvil gauges can use a dial or digital readout to show the interior width of a hole, though some gauges as mentioned below don’t use either.