be line and line.

37. Q. What is meant by steam lap?

A. The distance that the valve overlaps the live steam edges of the
steam ports when it is in the center of its travel over the seat. This
distance is measured at one end only, although the valve laps equally at
both ends.

38. Q. What is meant by exhaust lap? What by exhaust clearance?

A. Exhaust lap is the distance that the exhaust edge of the valve
overlaps the exhaust edge of the steam port when the valve is in
central position. Exhaust clearance is the opening between the exhaust
edge of the valve and the exhaust edge of the steam port with valve in
central position. If the valve has neither exhaust lap or clearance it
is said to be line and line.

39. Q. What is meant by release? What by compression?

A. Release is the point in the travel of the piston when the port is
opened. Compression is the distance the piston travels after exhaust
port closes before the live steam port opens. During this travel of the
piston the exhaust port is closed so the moving piston compresses the
steam left in the cylinder.

40. Q. With an indirect valve motion and outside admission valve, what
would be the position of the eccentric relative to the crank pin on that
side? What with a direct valve gear? What difference between outside
admission valve and inside admission valve as to this position?

A. With an indirect valve motion and an outside admission valve, the
go-ahead eccentric follows the crank pin with engine running ahead.
Without any lap or lead it would be a quarter of a turn or 90 degrees
behind the pin, but as all valves have lap and lead, the eccentric is
advanced or placed toward the pin enough to move the valve the amount of
the lap and lead. With a direct valve gear and an outside admission
valve, the eccentric will be a quarter of a turn or 90 degrees ahead of
the crank pin and advanced enough to move the valve the amount of the
lap and lead. With an inside admission valve and an indirect valve
motion, the eccentric will come the same as for an outside admission
valve and direct motion, or more than a quarter of a turn ahead of the
pin. With an inside admission valve and direct motion, as piston valves
are usually put up, the eccentric will follow the pin less than a
quarter of a turn.

41. Q. What effect would be produced upon the lap and lead by changing
the length of the eccentric rod?

A. Lap depends on the construction of the valve. A change of