A four-cycle air-oil cooled engine includes a cylinder head having a spark plug mounting hole formed therein, and a ring-like groove formed therein and surrounding the spark plug mounting hole. The cylinder head is formed with first and second co-planar sealing surfaces, disposed on a plane perpendicular to an axis of the spark plug mounting hole, and having an opening end of the ring-like groove disposed therebetween. The ring-like groove defines an oil jacket around the periphery of the spark plug mounting hole. A cover member having a flat surface opposed to the first and second flat sealing surfaces of the cylinder head is secured to the cylinder head such that a single gasket is interposed between the first and second sealing surfaces and the flat surface of the cover member.