The ST 102 is a 4 piece set of small hole gauges. These tools are used to determine the diameter of holes in various parts to a precise measurement. Great for testing wear on precision holes etc. They can measure from 1/8' up to 1/2". Each gauge has an expanding end that is controlled by a thumbscrew. Place the gauge into the hole and expand until it fits and then remove the gauge and measure the diameter with a dial caliper or micrometer.