These silver-plated hex-nuts are for non-plated 1/4-28 stainless steel bolts. They are commonly used with clearance hole 2.125 (2-1/8) and 2.75 (2-3/4) inch CF Conflat flanged fittings. These confalt flages can be pumped down to 10-13 torr (10-11 Pa) and can be heated to 450 °C for bake-out. North American flange sizes are given by flange outer diameter in inches: 1-1/3 ("mini conflat"), 2-1/8, 2-3/4, 4-1/2, 4-5/8, 6, 6-3/4, 8, 10, 12, 13-1/4, 14 and 16-1/2. In Europe and Asia, sizes are given by tube inner diameter in millimeters: DN16, DN40, DN63, DN100, DN160, DN200, DN250.