These are a range of gauges that are used to measure a bore's size, by transferring the internal dimension to a remote measuring tool. They are a direct equivalent of inside calipers and require the operator to develop the correct feel to obtain repeatable results.
The gauges are locked by twisting the knurled end of the handles, this action is performed to exert a small amount of friction on the telescopic portions of the gauge (the smaller diameter rods found at the T head of the gauge). Once gently locked to a size slightly larger than the bore, the gauges are inserted at an angle to the bore and slowly brought to align themselves radially, across the hole. This action compresses the two anvils where they remain locked at the bores dimension after being withdrawn.
A set of tools, shown in the image to the right, which cover the smaller sizes — 3 mm (0.125") to 13 mm (0.5").
They require a slightly different technique to the telescopic gauges, the small hole gauge is initially set smaller than the bore to be measured. It is then inserted into the bore and adjusted by rotating the knurled knob at the base, until light pressure is felt when the gauge is slightly moved in the bore. Measurement is again by external means.
The gauges shown in the image cover
This combination allows the bore size to be accurately read from the instrument with the minimum of effort. The fixed end of the anvil piece has pressure fingers on either side of it that assist correct placement in the bore, this reduces the reliance on feel by the operator, as required when using the telescopic gauge.