A large bore measuring system includes a modular and extendable bore gage with analog and digital readouts, also includes a settable set-master. The modular bore gage is designed to measure two sets of large bore diameters. A first set of large bores is accomplished by using a modular bore gage design described in this disclosure. The second set of larger bores is accomplished by adding attachments therein extending the gage for the larger range of bore diameters. A set-master is used for both sets of large bores.