Abstract: A cooling system has a coolant path which is external to the motor housing of the motor. The external coolant path is connected across the inlet and outlet of the internal coolant path through which coolant is normally conveyed to cool the hot zones of the motor. A closed circuit is thus formed. The external coolant path includes a heat exchanger arranged to place the coolant in heat exchange relationship with water in which the motor runs. It may also include a header tank for pressure control and topping up purposes. The external path can be in kit form for conversion of existing motors. The motor may be an inboard or outboard motor.