These are CF 3-3/8 inch tapped for 1/4 inch NPTF, Non-Rotatable fittings. ConFlat (CF) flanges are used for ultra-high vacuum applications. These fittings use a copper gasket and knife-edge flanges to achieve the high vacuum seal and can operate 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"), 2¾, 4½, 6, 8, 10, 12, 13¼, 14 and 16½. In Europe and Asia, sizes are given by tube inner diameter in millimeters: DN16, DN40, DN63, DN100, DN160, DN200, DN250. These vacuum fittings are made of corrosive resistant stainless steel. Sold as ONE EACH.