Cooling system structure for vehicular water-cooled internal combustion engine

A cooling system structure for a vehicular water-cooled internal combustion engine, comprising:

• a cylinder section constituting, together with a crankcase for rotatably supporting a crankshaft, an engine main unit, said cylinder section includes a cylinder body that projects upwardly from the crankcase, a piston operatively connected to the crankshaft is slidably fitted within the cylinder body and a cylinder head is connected to the cylinder body;
• water jackets are formed in the cylinder body and the cylinder head, respectively;
• a radiator is disposed forwardly of the engine main unit;
• a water pump is disposed on the engine main unit, the water pump circulates coolant through the radiator and the water jackets, the cooling system structure for a vehicular water-cooled internal combustion engine;
• said water jackets are respectively formed so as to allow coolant to flow through the cylinder body and the cylinder head independently of each other;
• and
• a thermostat is disposed on a front wall surface of the cylinder section, the thermostat being capable of selecting a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head directly communicate with each other or a condition in which the water jacket in the cylinder body and the water jacket in the cylinder head communicate with each other via the radiator,
• wherein the coolant flow is circulated from the inlet port of the cylinder head, at the outer side wall, to the outlet port of the cylinder body.