the outer surface of coupling housing 34 is extended and internally threaded at 108 near the end 110 thereof threads 108 mesh with threads 112 of internal plug load adaptor 114 adaptor 114 is provided with an arcuate surface 116 surface 116 is designed to register with arcuate outer surface 118 of integral flange 120 of tube adaptor 50 internally generated pressures can be absorbed to a greater degree with this embodiment in that forces tending to move ducts 12 and 14 axially away from each other are opposed by surface 118 bearing against surface 116 the arcuate shape of surfaces 116 and 118 allows for angular rotating such as with embodiments 1 and 2 about 34 o 34 ring 52 to accommodate angular misalignment of ducts 12 and 14 adaptor 114 is also provided with a knurled surface 113 to accommodate a strap like wrench referring to fig4 and 5 there is shown a fourth embodiment of the present invention this embodiment is much like the second embodiment fitted with a self locking device this embodiment uses the same numerals to identify corresponding parts of embodiments 1 and 2 retaining collar 26 has an integral extended portion 130 which extends past threaded area 33 of coupling housing 34 portion 130 has serrations 132 on the internal surface thereof a lock latch member 134 is provided with an upwardly extending latch portion 136 having an upper wedge shape latch 136 is designed to fit within serrations 132 the wedge shape is provided to allow the serrations to slip over the latch 136 when the retaining collar 26 is tightened in the direction of arrows 138 but provide a stop to prevent reverse movement of retaining collar 26 latch 136 is resiliently biased upward against serrations 132 by a leaf.