Vermont Gage Steel Go Plug Gage, Black Oxide, Tolerance Class ZZ, 0.138" Gage Diameter Supplied with a certificate of accuracy traceable to National Institute of Standards and Technology (NIST) Go (+) gage for measuring the lower limit of a hole Made from tool steel 60/64 Rockwell c scale (Rc) cold stabilized for durability Class ZZ (tolerance of .0002) for longer wear Black guard finish to inhibit corrosion and for indicating wear