Telescopic gauges are available in sets with a range of sizes to cover a variety of bores.
The springs inside a telescopic rod allow it to retract and expand, so it can move and fit into the bores.
The locking screw is attached to these springs, allowing the telescopic rods to be held at the exact measurement of the hole or bore.
A telescopic gauge does not have any measurements scale of its own, so it is referred to as a transfer measurement device. This means it needs to be used with a measuring device such as a micrometer to measure the exact size of a bore.