PURPOSE: To prevent reduction of output power by shortening the cooling water passage by at least 30mm from the open end of the exhaust pipe to the engine side, and providing a distance of at least 10mm from the exhaust pipe at the open end of the exhaust pipe.

CONSTITUTION: A cooling water passage 2 which is fabricated as one body with an exhaust pipe 1 is made shorter by a length of (a) (about 30mm) from the open end of the exhaust pipe 1 to the upper engine side, and at the opening of the cooling water passage 2 a rubber ring 2' is applied to connect it to the end of a cooling water supply pipe 3 which constitutes a part of the cooling water passage. The pipe 3 is connected to a conduit 6 through a pump 5. The pipe 3 is installed with a distance of (b) (about 10mm) maintained from the outer surface of the exhaust pipe 1 on the plane of the open end of the exhaust pipe 1. Consequentially, the pulsatory motion due to expansion of exhaust gas at the exhaust pipe opening will not be interfered by the cooling water passage, avoiding resultant output power reduction.