A header (10) for a harvesting machine with a movable cutterbar (16) comprises a belt drive mechanism connecting a stationary pulley (22) mounted on the frame (12) of the header and a relatively movable pulley (24) mounted on the movable part (18) of the header. The belt drive mechanism comprises two guide pulleys (28, 30), a belt (26) mounted along said pulleys (22, 24, 28, 30), and a pivot assembly (32) mounted on the auger shaft (16). The guide pulleys (28, 30) are mounted on opposite lever arms of the pivot (32) and said belt drive mechanism further comprises a tensioning mechanism.
