Abstract: A sealing system for an outlet of a polymer-lined compressed gas cylinder has a polymer liner outlet extending into a bore of a boss. An insert is engaged with the bore and a secondary angled seal, forming two primary seals between the insert and portion of the liner outlet. The first seal is an O-ring in the radial direction. The second seal is an angled seal, or beveled conical seal, of the polymer being compressed between the metal components of polar boss and the insert. The use of seal in two different directions compresses the polymer liner in two directions to prevent any possibility of gas leakage and/or seal extrusion under pressure. The angled seal surface prevents any reverse extrusion of the primary seal that might happen during cold temperatures or during repeated pressure cycles in long term service.