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.