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.

Twisting the knurled end of the handles locks the gauges; 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). To use, the gauge is inserted at a slight angle to the bore and gently locked to a size slightly larger than the bore while at that angle. Then, rocking the handle side-to-side, slowly move the handle across the bore to the other side. The rocking will first align the gauge with the bore axis and the act of moving the handle to the other side of the bore will bring it to the exact bore diameter. This action compresses the two anvils where they remain locked at the bores dimension after being withdrawn.