Abstract: A crop harvesting header for a combine wherein the header includes a transversely elongated frame having opposite side walls. A cutterbar extends between the side walls and is supported from the header frame for floating movement relative thereto as induced by varying ground profile. The cutterbar is connected to the forward ends of a plurality of transversely spaced runners pivotally connected to the header frame. One of the runners is disposed at an end of the header frame adjacent to one of the side walls. Reciprocating drive for the cutterbar is provided by a wobble drive mounted on the end runner and a belt drive therefor mounted on the adjacent side wall. A belt tightener pulley assembly is pivotally mounted on the side wall.