Seal-Lok O-Ring Face Seal Tube Fittings have proven to be very effective in eliminating leakage at the higher pressures found in today’s hydraulic systems. The Seal-Lok fitting is an O-ring face seal type fitting that consists of a nut, a body, an O-ring and a sleeve. Seal-Lok fittings are suitable for a wide range of tube wall thicknesses and are readily adaptable to pipe, inch or metric tubing and hose.