This tensioner gear drives the chain or belt that maintains proper tension in the header driveline. It is mounted on the tensioner arm and rotates on a bearing or bushing, keeping the drive loop tight so that power is transmitted without slip or slack. A worn or seized gear causes chain chatter, uneven wear on the sprockets, and eventual loss of drive to the header components. The original gear is machined to match the tooth profile and bore dimensions of the factory assembly, so it seats on the tensioner arm without modification. On reinstall, verify that the gear rotates freely and that the tensioner arm moves through its full range before locking the pivot bolt to the specified torque.