In a chain guide mechanism, fixed and movable guides are attached to a guide bridge by cooperation of bosses on the guides with boss-receiving holes formed in spaced-apart elements of arms of the bridge. A fixed guide is held against rotation by cooperation of a projection on its mounting boss with a notch in a boss-receiving hole in a guide bridge arm element. A base, extending from an intermediate location at which the arms come together has a pressing structure that engages bosses on a driving sprocket, and a groove for receiving teeth of the driving sprocket.