A telescoping gauge is a measuring tool with spring-loaded plunges used together with a micrometer to measure the inside of holes or bores. Telescoping gauges are made insets to measure from small to very large bores.
The telescoping gauge has a handle that is attached to two spring-loaded plungers. Using the handle , place the telescoping gauge into the bore . Release the lock screw on the handle. The spring-loaded plungers will come out and touch the side of the bore. Rock the gauge back and forth to be sure it is square in the bore. Turn the lock screw to lock the plungers in position. Remove the telescoping gauge from the bore. The exact size of the hole is found by measuring across the two ends of the plungers with an outside micrometer.
Inside calipers can be used to measure any gap that their legs will fit into, and because those legs are not hardened (and also because the tool is so inexpensive), they can be ground thinner to fit into really narrow cracks. Still, they are likely used most of the time to measure the inside diameter (I. d.) of holes. Another tool that can be used for exactly the same purpose is the telescoping gauge. Although its working ends are larger than those of an inside measurements as the I. d. of a snap ring groove, the telescoping gauge is definitely preferable for gauging simple round holes because of its potentially greater level of accuracy.
That advantage comes from the construction of the tool: It amounts to two straight legs, one of which telescopes into the other, working against a spring that tends to extend it. Part of the improvement in accuracy over calipers is provided by that spring, which automatically controls the gauging force. Another aid to accuracy is a handle attached exactly at right angles to the fixed leg.