The engine has a rocker box (22) with increased volume, fitted to the cylinder head (20), and a breather chamber (28) fitted on the rocker box (22). A gas blow-through opening (29) is provided in the rocker box to produce a flow connection between the rocker box and breather chamber. An oil passage (10) with an increased surface area connects the crankcase to the rocker box. A cover plate (34) is provided below the blow-through opening.
