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.

45. Q. How are metallic packing rings on piston rods and valve stems
held in place? What provisions are made for the uneven movements of the
rod?