This is a 6-piece Telescoping Bore Gauge Set, a precision tool kit used to measure the internal diameter of holes, slots, and cylinders ranging from 5/16" to 6" (8mm to 150mm). Each gauge features spring-loaded plungers that expand to fit a bore; once positioned, a knurled screw on the handle locks the measurement in place. Because these are indirect measuring tools, you simply lock the plungers at the desired width and then use a micrometer or caliper to read the exact dimension.