Telescopic feeler gauges
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 This action compresses the two anvils where they remain locked at the bores dimension after being withdrawn.
small hole gauges
Small hole gauges 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. The gauge is then removed and measured with a caliper or micrometer.