Abstract: A changeable sprocket wheel assembly enables the rapid and reliable mounting and demounting of sprocket wheels of different diameters on a film drive shaft by unskilled operators, while assuring that the sprockets are held with high precision. In one arrangement, an inboard sprocket wheel member (50, FIG. 2) receives the inboard portion (55) of a driven shaft in a close sliding fit and is lockable to the shaft, the inboard member having a small diameter land (52) with a ring of sprockets (54) thereon. An outboard sprocket wheel member (60) has the same size land with sprockets thereon, and fits around the outboard shaft portion (65) in a close sliding fit and is detachably fixable to the inboard member to abut it and rotate with it. When a larger diameter sprocket wheel is required, the outboard member (60) is removed and a large sprocket wheel element (70, FIG.