A first belt tensioner includes an arm (14, 112, 212), spring case (16, 114, 214), coil spring (12, 116, 216), and arm plate (118, 218). The arm (14, 112, 212) is adapted to support an idler pulley (20, 120, 220) and has a hook portion (22, 122, 222). The spring case (16, 114, 214) is disposed in the arm (14, 112, 212) and has a first spring seat (124). The spring (12, 116, 216) is disposed in the spring case (16, 114, 214). The arm plate (118, 218) has a second spring seat (132, 232). The spring (12, 116, 216) is longitudinally compressed between the arm plate (118, 218) and the spring case (16, 114, 214). The spring (12, 116, 216) is in torsion, with the end portions (18; 128, 130; 230) of the spring (12, 116, 216) seated against a corresponding spring seat (124, 132) and with the arm plate (118, 218) rotationally positioned under the hook portion (22, 122, 222). A method for assembling the first belt tensioner (110, 210) includes using the arm plate (118, 218) to longitudinally compress the spring (116, 216) and rotating the arm plate (118, 218). A second belt tensioner (10, 110, 210) includes an arm (14, 112, 212), spring case (16, 114, 214), coil spring (12, 116, 216), arm plate (118, 218), and pivot bushing (24, 148, 248).