6.21Mm Class Zz No-Go (Minus) Pin Gage (+.0000"/-.0002") With Black Guard™. Nist Traceable, Certificate Of Accuracy Included. The Black Guard™ Black Oxide Treatment  Penetrates The Surface Of The Gages Approximately .000040" - .000060" Deep. When The Gages Wear, From Use, The Black Will Become Silver, Showing The Gages Have Been Used And May Be In Need Of Calibration Or Replacement. The Black Guard™ Treatment Helps To Prevent Corrosion And Reduces The Coefficient Of Friction Of The Treated Surface Of The Gage. This Extends The Useful Gage Life. No-Go Pin (Minus) -.0002"/+.0000", 2" Long, Tool Steel 60/65 Hrc, 10 Microinch Finish Or Better, Round Within .0001" Or .0025Mm, Marked With Size And Direction Of Tolerance On Gages Over .0605", Certificate Of Accuracy Included (Printed On The Bag The Pin Is Packaged In) Vermont Gage Usa Part #912406210