Small hole gages are similar to Telescopic Gauge in that they are smaller in size, adjustable & have a rounded measuring member. A screw in the end of the handle is turned to expand the ball shape end in small holes and similar recesses. Then a micrometer is used to measure the ball end. The set of four gages is used to check the dimensions of small holes, slots, and grooves from 3 to 13mm in diameter.
Full Ball Type
To use a small hole gage fit the ball shaped end into the hole or slot. Expand the ball shaped end by turning the screw at the end of the handle while slightly rocking the gage. You will feel when the gage has expanded to the correct dimension. Gently remove the gage from the hole and take a measurement with an outside micrometer.