Well bore measurement tool

A well bore measurement tool for a well bore having a side wall, comprises: lower packer (6) for providing a lower seal in the well bore; upper packer (8) for providing an upper seal in the well bore; caliper means (10) for measuring a deflection of the side wall (4) of the well bore; and retainer means (9) for retaining the caliper means between said lower and upper packer means so that the caliper means is transportable into the well bore with said lower and upper packer means but so that said lower and upper packer means are longitudinally movable relative to the caliper means when said caliper means engages the side wall of the well bore. The caliper is preferably mounted on two springs in a free-floating manner between the upper and lower packers, and has radially extendible arms which are extendible in response to independent forces exerted thereon, as preferably derived from the movement of a carriage driven by a single motor contained in the caliper tool. A clutch mechanism is used to lock the arms to precision measurement transducers only after the arms have been moved radially outwardly a sufficient distance.