6 Pc Precision Telescoping Gage Set 5/16" - 6" Range T-Bore Hole Gauges w/ Pouch Range: 5/16" - 6" (8mm to 150mm), 6 pc set includes Stainless Steel Construction Spring loaded with locking knurled knob Gage expands in a hole to be measured (locks in place and then removed to determine final hole size using a micrometer or calipers). Contact points are hardened tool steel and precision ground in the end measuring radius