Abstract: Apparatus for forming a tight seal between a coupling and a tube of variable sizes. A cylindrical housing defining an opening therein is provided such that a plurality of jaws having gripping teeth and gear teeth are mounted within the housing by being rotatably secured on a pin. A sleeve having rack teeth for engaging the gear teeth of the jaws is provided for rotating the jaws such that the inward extension of the gripping teeth may be varied. A piston having a seal mounted thereon and a projecting member extending therethrough is slidably maintained within the cylindrical opening such that a joint may be formed between the tube and the coupling. The piston defines together with the housing an expansible chamber which may be filled with a pressurized fluid to move the piston against the tube to assure a tight seal. A limit switch may additionally be provided such that if the gripping teeth do not properly engage the exterior of the tube the piston will be displaced sufficiently to engage the limit switch.