_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.