Abstract: The tightener includes a contoured strip adapted to bear against the chain or belt and be pivotally mounted on a fixed support member. A wedge member slidably mounted on the support member is elastically biased by a spring and urges the contoured strip against the chain or belt. A guide rod extends through a bore in the wedge member and the spring is mounted on the guide rod. A plate slidably mounted on the guide rod between the spring and the wedge member along one of the edges of the plate and thereby create an eccentric force which jams the plate on the guide rod.