A gauge (1) for checking radial dimensions of a workpiece (60), e.g. the diameter of a crankpin orbitally rotating during grinding operations, includes a casing, a V-shaped reference device (4) and a rod that can move within the casing (2) with respect to the reference device, an end of the rod projecting from the casing and carrying a feeler (20) for touching the surface of the workpiece to be checked and transmitting the movements of the feeler to a transducer (22). A sealing system for preventing coolant, dust or other foreign matter from getting inside the casing includes a pneumatic conduit (6) for allowing compressed air to be blown from inside the casing to outside, and an internal seal that—in the non-operative condition—seals the opening through which the rod projects from the casing. According to an advantageous embodiment, the seal is fastened into a bushing guiding the rod and cooperates with a surface of the rod in order to get the tightness.