Our Super Flangeless PCTFE Ferrule is designed specifically to eliminate loosening of fittings due to tube twisting, hold tight even during vibration and delivers the highest pressure holding for flat bottom fittings that we offer. It is made from PTCFE delivering excellent chemical resistance. Our ferrule can be used with a 1/4-28 or M6 Flat-Bottom port configuration and is for 1.8mm OD tubing . It comes in green.