A cam shaft assembly (50), which engages a plurality of cam followers (51) which operate valves (52) of an internal combustion engine. The cam shaft assembly (50) has a rotatably driven central shaft (56) to which a first cam lobe (57) is attached. An outer tubular shaft (58) has attached to it a further lobe (59). The shafts (56 and 58) are angularly displaceable to angularly displace the lobes (57 and 59) to alter valve timing and dwell angle.