A cylinder head for an internal combustion engine has a body, a rigid center spacedly surrounded by the body, an annular outer valve seat on the body, and an annular inner valve seat on the center. A ring-shaped valve has an outer seat face and an inner seat face and is displaceable along an axis into a closed position bearing on the outer valve seat with the outer seat face and on the inner valve seat with the inner seat face. The inner valve seat or outer valve seat is slidable axially. A seal member set in a groove in the center or in the body engages the one slidable seat. At least one biasing member connected to the one slidable valve seat urges same toward the ring-shaped valve.