10. Yoke.
a. The steam cylinders and water cylinders must be connected by such a form of yoke as requires no packing, a metal to metal joint at this connection being considered necessary. The piston-rod stuffing box heads should concentrically fit the counter-bore of the yoke.
If packing is put into these joints, there is a chance of the steam and water ends getting out of alignment and leaking at the joint between cylinders and yoke.
11. Steam Ports.
a. The area of each exhaust steam passage, at its smallest section, must not be less than 4 per cent. of the area of the piston from which it leads.
This is a large increase over the size heretofore common, but indicator cards which we have taken from pumps of several different makes indicate this to be one of the points in which improvement is most needed to accommodate the high speeds at which fire pumps are always supposed to run, and this unrestricted exhaust aids very materially in giving steadiness to the jet of water.
b. Each admission port must be not less than 21⁄2 per cent. of area of its piston, and to avoid wasteful excess of clearance, these passages should not be bored out larger in interior of casting than at ends or passage.
c. The edges of the steam-valve ports must be accurately milled, or chipped and exactly filed to templets, true to line, and the valve seat must be accurately fitted to a plane surface, all in a most thorough and workmanlike manner and equal to high-grade steam-engine work.
d. To guard against a piston ring catching in the large exhaust ports, these ports must have a center rib cast with cylinder at cylinder wall. [See also Art. 13] d.