The surface- of the telescopic rod is smooth, and the curvatures of the measuring head is good.
Use the gauge to expand in the hole to be measured, lock it in place, and then take it-down, and measure with micrometer caliper to determine the hole size-.
The telescopic spring plunger in the bore grooves), can do the inner diameter detection, used with micrometer caliper, with knurled locking device.