Abstract: A portable power unit for unloading a silage wagon which unit is adapted to be conveniently mounted on the back of the truck used to tow the wagon. The power unit comprises a gasoline engine and a gear reduction unit that is driven by the engine, the reduction unit having a splined output shaft that is adapted to be connected to the self-unloading wagon to power its conveyor. The engine drives the gear reduction unit through a normally loose belt, and a clutch is provided that takes the slack out of the belt when the clutch is engaged. The belt pulley for the gear reduction unit has a brake disc connected to it which coacts with a caliper brake. The brake is actuated by the clutch when the latter is disengaged and operates to hold the brake disc and its pulley against rotation.