A camshaft assembly includes an inner shaft, an outer tube surrounding and rotatable relative to the inner shaft, and two groups of cam lobes mounted on the outer tube. The first group of cam lobes is fast in rotation with the outer tube and the second group is rotatably mounted on the outer tube and connected for rotation with the inner shaft by means of pins that pass with clearance through slots in the outer tube. A spring is incorporated in the camshaft assembly to bias the inner shaft relative to the outer tube towards one extreme of its angular range.