(57) In order to adjust the extension of a twin-section telescopic structure, normally compression loaded, the sections themselves, forced to stretch by an inner coaxial spring, are connected to each other by an inner structure that is also coaxial, can be adjusted in extension and is made up of a cylinder-piston unit of a completely oil filled equal section inner chambers type. One stem of such cylinder+ piston unit is fixed to the free end of one of said sections, while the body of the cylinder - piston unit is fixed to the free end of the other telescopic section with interposition of bearings. A cylindrical box is fitted on or in proximity of the piston of the hydraulic unit. This box intercepts special holes through which the pressure chambers of the same unit can be interconnected. The box is forced to close by a spring and can be remote controlled by means of a control rod that passes axially through the stem fixed to one of the telescopic sections and sticks out of that section for the necessary operation.