Abstract: A ring gauge includes an entry zone followed by a measuring zone defined by a guide cylinder in which is installed an air jet measuring system. The entry zone includes a first region exhibiting an opening substantially larger than the opening of the guide cylinder then a second region including a spring support guide structure which defines a circle the diameter of which is equal to the diameter of the guide cylinder opening, and finally a third tapered region having an entry diameter substantially greater than that of the guide cylinder opening and an exit diameter equal to that of the guide cylinder opening. The gauge is employed for checking the outer diameter of cylindrical parts which may penetrate therein without jamming.