Abstract: A cylinder head 12 showing a guideway in which is located a rotatable valve 10, 12 comprising a fluid port operable to effect fluid communication between a cylinder and a fulid manifold in the guideway, whereby rotation of the valve effects alignment of the fluid port with the combustion chamber of a cylinder 4 to enable fluid flow between the valve 10, 12 and a cylinder 4, and wherein the cylinder head 2 further comprises a seal 22 which, in use, is movable from a first, non-sealing position in which the seal 22 is biased away from the valve 10, 12, and a second, sealing position in which the seal 22 is biased onto the valve by gaseous pressure from within a cylinder 4.