An adjustable hydraulic coupling device allows simultaneously mounting of different parts of a downhole drilling tool to the drill string near its upper and lower extremities. This top- and bottom-mounting is made to points on the drill string separated by an indeterminate length. The coupling device allows a mechanical and sealed fluid connection between two portions of the tool, and allows assembling of both portions of the drilling tool into one functional unit, and allows both axial translation and rotational motion therebetween. A cylindrical protruding tube is attached to the portion of the drilling tool mounted at its upper extremity while a mating cylindrical tube is attached to the part of the drilling tool mounted near the bottom. During assembly, the smaller tube enters the larger tube and provides a sealed fluid path and also allows for the engagement length of the assembly to vary as required.