An internal combustion engine comprising at least a crank case, which houses a cylinder, and a cylinder head, which is adapted to be coupled to the crank case and comprises: - at least one exhaust port and at least one intake port, - an injector arranged in an approximately central position of the cylinder head between an intake valve of said intake port and an exhaust valve of said exhaust port, and - a liquid cooling circuit, wherein the liquid cooling circuit comprises: - at least one inlet aperture and at least one outlet aperture communicating with the outside of the engine; - a lower cooling chamber housed in said cylinder head in a position adjacent to a flame deck of said cylinder head; - an upper cooling chamber housed in said cylinder head above said lower chamber, so that said lower chamber is sandwiched between said upper chamber and said flame deck; wherein said inlet aperture and said outlet aperture are arranged on a same side of the engine, and wherein said upper chamber and said lower chamber mutually communicate with one another, so that the cooling liquid runs in a substantially U-shaped path moving, at first, through said upper chamber and, then, through said lower chamber or vice versa according to a direction that is transverse to a relative crankshaft; wherein said inlet aperture and said outlet aperture are arranged on a same side of the engine, where said exhaust port is arranged; characterized in that said U-shaped path joins said exhaust port to said intake port; and wherein said upper chamber and said lower chamber are mutually communicating by means of - at least one main aperture, which is arranged between said intake valve and an outer side of the engine opposite said inlet aperture. - at least one exhaust port and at least one intake port, - an injector arranged in an approximately central position of the cylinder head between an intake valve of said intake port and an exhaust valve of said exhaust port, and - a liquid cooling circuit, - at least one inlet aperture and at least one outlet aperture communicating with the outside of the engine; - a lower cooling chamber housed in said cylinder head in a position adjacent to a flame deck of said cylinder head; - an upper cooling chamber housed in said cylinder head above said lower chamber, so that said lower chamber is sandwiched between said upper chamber and said flame deck; wherein said inlet aperture and said outlet aperture are arranged on a same side of the engine, and wherein said upper chamber and said lower chamber mutually communicate with one another, so that the cooling liquid runs in a substantially U-shaped path moving, at first, through said upper chamber and, then, through said lower chamber or vice versa according to a direction that is transverse to a relative crankshaft; wherein said inlet aperture and said outlet aperture are arranged on a same side of the engine, where said exhaust port is arranged; characterized in that said U-shaped path joins said exhaust port to said intake port; and wherein said upper chamber and said lower chamber are mutually communicating by means of wherein said inlet aperture and said outlet aperture are arranged on a same side of the engine, where said exhaust port is arranged; characterized in that said U-shaped path joins said exhaust port to said intake port; and wherein said upper chamber and said lower chamber are mutually communicating by means of - at least one main aperture, which is arranged between said intake valve and an outer side of the engine opposite said inlet aperture.