Abstract: A mechanical excavator provided on a tractor or other vehicle and which comprises a main boom, which at its lower end is pivotally connected to a king post assembly for movement up and down in a vertical plane, a dipper arm pivotally connected to the outer end of the boom for movement in a vertical plane and carrying or being adapted to carry a bucket or like tool, the king post assembly providing for pivotal slewing movement of the main boom about a vertical axis so that the main boom can be rotated from one side to the other of the center line of the vehicle, the king post assembly being mounted upon a guide frame extending transversely of the center line of the vehicle, the king post assembly being moveable along the guide frame, and fixing clamps for releasably fixing the king post assembly relative to the guide frame, wherein a fluid operated ram connected between the king post assembly and an anchor which is adapted to be fixed relative to the guide frame whereby extension or contraction of the fluid op