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"While the crank is moving from the position, steam is being admitted to the head end and being exhausted from the crank end. The inside lap being less than the outside lap, causes the exhaust to continue longer than the admission." —Derr, 1911

Steam Engine Valve with Lap Where Steam is Admitted to the Slide Valve

"While the crank is moving from the position, steam is being admitted to the head end and being exhausted…