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Telescopic gauges, also known as snap gauges and telescoping bore gauges, are used to measure the diameter of a hole, groove, or slots. They are a T-shaped tool with two telescopic rods that stretch out to the internal wall of holes and are locked in position using a locking screw. The span of the telescopic heads is then compared to a calibrated micrometer to determine the actual diameter size. Telescopic gauges are commonly used by automotive mechanics in measuring crankcase bore sizes and by machinists.