A while back there was a question posed about lap and lead on steam locomotives. I can't find the thread, so I thought I would post this as a new thread.
The information is taken from "Basic Steam Locomotive Maintenance" published by Simmons - Boardman Publishing Company.
31. In order that steam may be used expansively, the steam or admission edges of the valve are extended. This addition to the valve is called STEAM LAP, and is illustrated at A in Fig.15. Steam lap is defined as the amount that the steam (admission) edge of the valve covers the steam ( admission) edge of the steam port with the valve in the center of its seat. It is measured in inches and fractions of an inch. It must be equal on both ends of the valve, because it is measured with the valve in its central position.
32. Further expansion of the steam can be obtained by adding to the exhaust edges of the valve. This is shown at B, Fig. 16; such addition is known as EXHAUST LAP. It can be defined as the amount that the exhaust edge of the valve covers the exhaust edge of the steam port, with the valve in its central position. It is measured the same as steam lap and must of necessity be equal on both ends of the valve.
53. It was stated in paragraph 44 that the admission of the steam occurred shortly before the end of the return stroke, at the end of the return stroke, or else a moment after the beginning of a new cycle. This requires some explanation. The point at which the admission actually does occur depends on the setting of the valve. If, with the locomotive on dead center, that is, with the piston at the extreme end of its stoke, the valve setting is such that the valve is just ready to open to admit steam, it is said that the valve is set LINE AND LINE.
54. If, with the locomotive on dead center, the valve is already open a certain amount, the opening is spoken of as the lead of the valve. Lead is the amount in fractions of an inch that the steam edge of the valve has uncovered the the steam edge of the steam port with the locomotive on the dead center. Fig. 5 shows the piston at the extreme end of its travel ready to start a new stroke. The amount that the valve has uncovered the steam port is the lead.
