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 2-1/2 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._ 12. STEAM-CLEARANCE SPACE. _a._ Clearance (including nut-recess, counter-bore, and valve passages) must not exceed 5 per cent. for contact stroke or about 8 per cent. for nominal stroke (_i. e._, contact stroke should overrun nominal stroke not more than one-half inch or not less than one-fourth inch, at each end). _b._ The clearance space between face of piston and cylinder head must be reduced to smallest possible amount, and these contacting surfaces be flat, without projections or recesses other than the piston rod nut and its recess. Some makers, with the idea that a fire pump need not be economical, have not taken pains to keep these waste spaces small. Securing small clearance costs almost nothing but care in design, and is often of value, since at many factories boiler capacity is scant for the large quantity of steam taken by a fire pump of proper size. 13. STEAM PISTONS. _a._ May be either built up or solid, as maker thinks best. It is believed that “solid” (cored) pistons with rings “sprung in,” are for fire-pumps much preferable to built-up pistons, since follower bolts _do_ sometimes get loose. _b._ Piston must not be less than four inches thick between faces. If solid, walls should be not less than 1/2 inch thick, and special care should be given to shop inspection to determine uniformity of thickness. _c._ If built up pistons are used, involving follower bolts, such bolts must be of best machinery steel, with screw thread cut for about twice the diameter of the bolt and fitting tightly its whole length. _d._ The width of each piston ring must exceed the length of the large exhaust port by at least 1/4 inch. This is to avoid the possibility of piston ring catching in the port. See also Art. 11 _d._ 14. STEAM SLIDE VALVES.