Telescopic gauges are not suitable for measuring holes below 9 mm. For measuring smaller holes and slots, small hole gauges are used.
Construction of Small Hole Gauge
A small hole gauge consists of a tube having holes on the opposite sides at one end where hardened balls are fixed. The other end of the tube has an external thread. A screwed thimble is fixed with the threaded tube. A plunger with a tapered end, and spring-loaded, is inserted in the tube and tightened with the screwed thimble.
At the end of the thimble a knurled handle is fitted. While rotating the knurled handle in a clockwise direction the plunger moves forward up, and pushes the balls out to contact the surfaces.
A small hole gauge is an instrument used for indirect measurement, while a micrometer is usually used for measuring the sizes directly.
Small hole gauges are available in a set of 4 numbers to measure holes from 3 mm to 13 mm.