A header for a harvesting machine with a movable cutterbar includes a belt drive mechanism connecting a stationary pulley mounted on the frame of the header and a relatively movable pulley mounted on the movable part of the header. The belt drive mechanism includes two guide pulleys, a belt mounted along said pulleys, and a pivot assembly mounted on the auger shaft. The guide pulleys are mounted on opposite lever arms of the pivot and said belt drive mechanism further includes a tensioning mechanism.