_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.