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 the eccentric rod would not effect it, but would widen the port opening at one end of the travel and reduce it at the other.  It should be equal at both ends.  Lead is controlled by the position of the eccentric on the axle and it must be equal at both ends.  Changing the length of the eccentric rod from the proper one does not really affect the lead, because no proper measurement can be made until lead is equal at both ends.  Therefore improper length of eccentric rods varies the port opening at the beginning of the stroke of the piston at both ends.

42.  Q. Why are eccentric rods made adjustable?

A. In order to change their length to make adjustment of the valve gear not as easily made in other ways.

43.  Q. Why is it necessary to keep the cylinders free from water?

A. In order to avoid damaging valves and cylinders, to insure perfect lubrication and obtain the most efficient service from the locomotive.

44.  Q. Where is the piston rod packing located?  Where cylinder packing?

A. Piston rod packing is usually soft metallic rings located inside of a gland at the back end of cylinder and around the rod.  Cylinder packing rings are usually cast iron, placed around the piston head and bearing against the walls of the cylinder.