An o-ring boss seal is a technique for joining two fluid-carrying pipes, hoses, or tubing.
This system differs from others in that a nut is tightened over an o-ring in a chamfered area, creating a fluid-tight seal.
Under the high fluid pressures commonly seen in hydraulic systems, a leak or failure of the connection is quite dangerous and could lead to loss of life.
Most threaded systems, such as NPT, have a seal provided by a taper in the thread, so it is difficult to orient both ends of the hose, pipe or tube so that it is not twisted.
In the o-ring boss system, this problem is eliminated because the threads do not seal the connection and therefore can be rotated at least a full revolution before they are sealed while maintaining a proper mechanical connection.