A belt tensioning apparatus for a combine or harvester header has a frame with two spaced apart longitudinal support members and at least one transverse member attached to each of the longitudinal support members. An end roller support bracket has two bracket extensions extending longitudinally and disposed to slidingly engage with bracket extension receivers attached to the longitudinal support members. A push rod is longitudinally oriented and slidingly mounted on the transverse member intermediate to the longitudinal support members. The push rod is attached to the end roller support bracket and mounted to have a retracted position and an extended position. An actuator is attached to the longitudinal support members and exerts a substantially uniform force to the push rod along a range of travel of the push rod between the retracted position and the extended position. The actuator may be a scissors assembly or a hydraulic cylinder.