Abstract: An extensible telescopic machine part such as a telescopic boom for an excavator has three extensible, telescopic elements. One of the telescopic elements carries a working unit at its free end remote from the other telescopic elements. The working unit is supplied with energy through flexible energy transmission lines. Actuators for causing telecopic relative movement of the telescopic elements are provided. The energy transmission lines extend inside the machine part and are arranged in a carrier track. The actuators are arranged off axis with respect to the telescopic elements to provide space for the carrier track. The carrier track extends from a forward end of the outer telescopic element to the rear end of the inner telescopic element, The carrier track is guided between the outer and median telescopic elements and forms a bight around the rear end of the inner telescopic element.