A hydraulic drive device having a pump and motor is disclosed, where the pump swash plate includes a pair of structural ribs and the housing has a pair of thrust ribs on an inner surface. Only one thrust rib is in contact with one of the structural ribs at a time, to permit arcuate rotation of the swash plate while limiting axial movement of the swash plate. The axes of rotation of the swash plate trunnion, input shaft and output shaft are in a common plane. A rotary bypass shaft for engaging transmission check plugs includes an external bypass arm. A pair of detents are externally formed on the housing, and the bypass arm includes separate pairs of openings to engage the pair of detents when the bypass arm is in different positions.