Inlet valves, with inlet cages, in inlet ports in a cylinder head, and outlet valves, with outlet cages, in outlet ports in the cylinder head, comprise mating combinations of elements which exhibit optimum and proper seating together. The bores and the cages have configurations which, upon elements of one of the combinations being substituted for elements of the other combination, will prevent the seatings from occurring. Consequently, only the correct elements, per combination, will join together and nest in the correct bore and, in this way, the seating of an inlet valve in an outlet bore, or vice versa, is avoided.