How do telescopic bore gages work? A telescopic bore gage measures the size of a bore through indirect methods. Essentially, the telescopic bore gage is used to take the size of a bore, and then an external tool, such as a caliper or a micrometer, is used to measure the output of the gage. The head of the bore gage is extended at an angle within the bore and locked into place. The extended head is the part that is measured to get the final output. Very similar to inside calipers, which can also be used to measure bore diameter, telescopic bore gages have the added advantage of being able to be locked in place during the measurement process, thus ensuring higher accuracy. Telescopic bore gages are used by mechanics and anyone in metrology that needs to find the interior diameter, radius, or circumferences of a pipe or a hole.