the outlet side of the engine the main pipe 2 comprises a two way valve 4 which allows the flow rate of liquid coolant through the main pipe 2 to be suspended reversibly the siting of this valve 4 makes it possible to avoid having to site a component at the interface between the pump 5 and the engine 1 that is to be cooled in addition thanks to the tapping of the secondary pipe 3 off the main pipe 2 upstream of the engine inlet the closing of the valve 4 allows the flow rate of liquid coolant through the main pipe 2 to be stopped while at the same time maintaining a continuous flow through the secondary pipe 3 at the cooling chamber the main pipe 2 may at its end positioned directly downstream of the pump 5 comprise a distribution gallery 2 a which runs alongside at least part of the crankcase cylinder block and or the cylinder head of the engine emanating from this distribution gallery 2 a are several accessory pipes 2 b which contribute directly to the cooling of the engine 1 by supplying various zones of the cooling chamber the distribution gallery 2 a itself plays no direct part in the cooling of the engine 1 which means that the liquid coolant passing through it comes directly from the pump 5 and has not yet exchanged heat with the engine 1 the heat exchange capacity of the liquid coolant collected by the secondary pipe 3 is therefore optimal the tapping of the secondary pipe 3 can then be directly off the length of the distribution gallery 2 a according to a preferred embodiment the tapping of the secondary pipe 3 off the distribution.