A bore is a generic term used to describe a hole that has been enlarged and held to close tolerances of size and form. While boring is the usual process to enlarge a hole, the term ‘bore’ also applies to hole that has been enlarged by other processes, such as reaming or grinding.
A cylinder, on the other hand, is a series of circles of the same size. Straight bores, holes, and outside diameters are all cylinders. In a cylinder, each circle’s center point lies on an imaginary line called the axis. Each circle that makes up the cylinder lies in a plane perpendicular to this axis.
A bore gauge, as the name implies, is a tool used to measure bores. When used to measure a bore, the bore gauge is actually measuring the diameters of some of the circles within the cylinder. To accurately measure the cylinder, the bore gauge must measure across each circle through its center point, and also perpendicular to the cylinder’s axis.