To use these gauges select one that has the range of your bore dia. Loosen the handle, push in the two ends together (only one actually moves), insert it into the bore, make sure that the handle is vertical to the centerline of the bore, loosen the handle which will allow the end to spring out to the bore's dia., tighten the handle, cock the tool sideways slightly, and slide it out of the bore. Measure the gauge length with a micrometer and that will give you the dia.