Abstract: A downhole tool connectable to a drill string by a connection atop the tool includes a mandrel for connection with the drill string, the mandrel being rotatable within a body. A plurality of blades are individually extendible radially from the body to engage the wall of a well bore, the radial position of the blades being adjustable between a first retracted position and a second extended position to position the centre line of the mandrel at a desired position relative to the longitudinal axis of the well bore. A positioning system is provided to hold each of the blades at the retracted position, the extended position or at any intermediate position therebetween.