The diameter of Corliss valves used for the admission of steam to engine cylinders when the diameters of the cylinders are known may equal one-eighth the diameter of the steam cylinder plus 2 in., while the diameter of similar exhaust valves may equal one-sixth the diameter of the cylinder plus 2 in. Thus, for a cylinder 21 in. diameter, the steam valve should be 24/8 + 2 = 5 in. diameter, and the exhaust valve 24/6 +2 = 6 in. diameter.