_a._ The diameter of the disc of rubber forming the valve must not be
greater than 4 inches or less than 3 inches. Three and a half inches
diameter is probably the most favorable size, but is not insisted upon.
There is some confusion between different shops about designating
size of valves. The practice is here adopted, which is much the most
widely used, of naming the diameter of the disc of rubber which
covers the ports, and it is hereby specified that this shall be about
1/2-inch greater than the diameter of the valve-port circle which
it covers, thus affording about 1/4-inch overlap or bearing for the
rubber disc all around its edge.
If valves are larger than 4-inch there is an increased tendency to
valve-slam at the very high speed at which the pump is designed to
run, and if valves are smaller than 3 inches diameter the greater
number tends to unnecessary multiplication of parts, and the ports
being so small are a little more liable to become obstructed by
rubbish.
_b._ The thickness of the rubber valve must in no cases be less than
5/8-inch.