ClipArt ETC
"If the inside lap is increased, the valve must move farther before released occurs and the crank angle would be greater than illustrated. On the return of the valve to the dotted position, the port will close earlier and make an earlier compression. Decreasing inside lap will cause earlier release and later compression." —Derr, 1911

Steam Engine Valve with Lap Steam Where Slide Valve Moved to Open Position

"If the inside lap is increased, the valve must move farther before released occurs and the crank angle…