Abstract: A cylinder head assembly for a cylinder of a four stroke internal combustion engine, including an intake rotor assembly that includes an intake rotor body, a first intake rotor shell portion, and a second intake rotor shell portion, and is operable to be rotatably received in at least one through bore of a cylinder head member. An exhaust rotor assembly includes an exhaust rotor body, a first exhaust rotor shell portion, and a second exhaust rotor shell portion, and is operable to be rotatably received in the at least one through bore of the cylinder head member. At least one of the first and second intake rotor shell portions or the first and second exhaust rotor shell portions are operable to be urged outwardly towards or against an interior surface of the at least one through bore of the cylinder head member so as to create a seal therebetween.