A cooling fan assembly has an inner surface provided with a guide body for guiding a cooling air towards a first engine cylinder, positioned on a leading side with respect to the direction of rotation of the cooling fan assembly, and a second engine cylinder, positioned on a trailing side, after the cooling air has been separated. The guide body includes an upright wall protruding from the inner surface of the fan casing in an axial direction of the cooling fan assembly and extending radially outwardly and spirally from the inner surface of the fan casing as it progresses from the trailing side towards the leading side, and a ceiling wall connected with a projecting end of the upright wall and extending inside the spirality and operable to guide the cooling air inside the spirality of the upright wall towards the first engine cylinder.