Abstract: An eccentric pivot arm tensioner capable of automatic belt tensioning and substantially simultaneous operating range indexing during installation. The tensioner comprises a stop plate and cooperating lock plate. The tensioner comprises a pivot arm having a pivot arm member that cooperatively engages a base plate member. In a first, or transit mode, the pivot arm member temporarily engages the base plate member to put a torsion spring and thereby the pivot arm in a preloaded condition. The lock plate is engaged with the stop plate. As a mounting fastener is engaged into a mounting surface the pivot arm member is automatically disengaged from the base plate member by an axial movement of the pivot arm, thereby allowing the pivot arm to rotate and thereby impart a torsion spring force to a belt, i.e., tensioning the belt in an operating position.