An improved screw-on assembly for coupling line connectors with spouts has been invented. The invention can be used with bag-in-box packaging or with a variety of other applications requiring fluid coupling. In one embodiment, threading is accomplished by mating internal threads on a line connector with flexible tabs located on the spout. Deformation of the tabs during threading provides a restorative force that stabilizes the assembly during threading. Cooperation of the deformable tabs with structures near the end of the thread provides for self-locking of the assembly. In addition, the tabs are situated on a flange, providing compatibility of the present invention with prior art line connectors.