Abstract: A cylinder head (12) comprising 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 fluid 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).