_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 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._