What is lead? Lead is space or opening of port on steam end of
cylinder, when engine is on dead center. (Dead center is the two points
of disc or crank wheel at which the crank pin is in direct line with
piston and at which no amount of steam will start the engine.) Different
makes of engines differ to such an extent that it is impossible to give
any rule or any definite amount of lead for an engine. For instance, an
engine with a port six inches long and one-half inch wide would require
much less lead than one with a port four inches long and one inch wide.
Suppose I should say one-sixteenth of an inch was the proper lead. In
one engine you would have an opening one-sixteenth of an inch wide and
six inches long and in the other you would have one-sixteenth of an inch
wide and four inches long; so you can readily see that it is impossible
to give the amount of lead for an engine without knowing the piston
area, length of port, speed, etc. Lead allows live steam to enter the
cylinder just ahead of the piston at the point of finishing the stroke,
and forms a "cushion," and enables the engine to pass the center without
a jar. Too much lead is a source of weakness to an engine, as it allows
the steam to enter the cylinder too soon and forms a back pressure and
tends to prevent the engine from passing the center. It will,
therefore, make your engine bump, and make it very difficult to hold the
packing in stuffing box.
Insufficient lead will not allow enough steam to enter the cylinder
ahead of piston to afford cushion enough to stop the inertia, and the
result will be that your engine will pound on the wrist pin. You most
likely have concluded by this time that "lead" is no small factor in the
smooth running of an engine, and you, as a matter of course, will want
to know how you are to obtain the proper lead. Well don't worry
yourself. Your engine is not going to have too much lead today and not
enough tomorrow. If your engine was properly set up in the first place
the lead will be all right, and continue to afford the proper lead as
long as the valve has not been disturbed from its original position; and
this brings us to the most important duty of an engineer as far as the
engine is concerned, viz: Setting the Valve.