A gearbox assembly comprising a casing defining a cavity for housing gears of an engine-driven apparatus, the casing having at least one substantially flat exterior surface for receiving a cooler plate, and a cooler plate mounted to the casing at the substantially flat exterior surface, the cooler plate isolated from the cavity by the flat exterior surface, the cooler plate having a channel for receiving a liquid for cooling the casing and at least one port in communication with the channel. At least one further embodiment includes a gearbox assembly where the channel is aligned with the exterior surface for supplying liquid in contact with the surface. The invention also includes a cooler plate for attachment at a closed exterior surface of a gearbox in order to supply cooling liquid in contact with the gearbox surface to cool the gearbox.