A tool for installation of units in connection with oil wells, comprising a housing (1) with an attachment device (12) for attachment to a well installation, a telescopic device (30, 31, 32) for extension of the tool, a torsional force transmission device (13) for rotation of an internal movable element (2) of the tool. The internal movable element (2) comprises a pressure face (21), which in an operative condition is exposed to a well pressure and a counter-pressure face (22), with a sealing arrangement (26) between the housing (1) and the internal movable element (2) provided between these pressure faces (21, 22) and devices (23) which are arranged in such a manner that a pressure can be applied to the counter-pressure face.