A tightener for a belt drive cooperates with the belt in the presence of oil, and has a pin with a first axis; a base plate integral with and substantially perpendicular to the pin; a cam arm connected in a rotary manner to the pin and movable with respect to the base plate; a contact member carried by the cam arm; and an elastic member connected to the cam arm to activate the contact member against the belt. The tightener also has a friction member made of oil-resistant material and axially supporting the cam arm on the base plate; and axial elastic means for loading the friction member against the base plate.