A zero turning radius mower has an internal combustion engine mounted transversely on a pair of longitudinal frame members behind an operator seat and a pair of rear drive wheels. The engine rotates a flywheel on a horizontal axis perpendicular to the longitudinal frame members. A gearbox is mounted to one of the longitudinal frame members, with a horizontal input shaft driven by the engine and a vertical output shaft engaging one or more rubber belts. A vent passage is provided through the output shaft of the gearbox and between a hub of a rotary fan and the output shaft.
