To twice the lap add twice the width of a steam port in inches, and the sum will give the stroke required.

Expansion by lap, with a slide-valve operated by an eccentric alone, cannot be extended beyond 1/3 of the stroke of a piston without interfering with the efficient operation of the valve; with a link motion, however, this distortion of the valve is somewhat compensated. When the lap is increased, the throw of the eccentric should also be increased.

When low expansion is required, a cut-off valve should be resorted to in addition to the main valve.