Tube nuts are used in all types of automotive hydraulic lines, including brake fuel, oil, and transmission. To make a connection, place nut over the tubing to be flared, then flare . The tubes flare will be clamped between the nut and the flare seat when tightened. Tube nuts are formed to work with either SAE Inverted Double Flare (Domestic and Japanese vehicles) or ISO Bubble Flare (European and British vehicles). In either case, the tube nut must be selected to match the end form of the tube or port it will be mating to or a proper seal will not be possible.