gine shown has a stroke of 7/8 inch.
Therefore, the crank-pin _K_ on the crank-disk _N_ must be placed 1/2 of
7/8 or 7/16 inch from the center of the disk _N_, so that when this disk
makes one revolution, the piston will move 7/8 inch in the cycle. Thus
it will be seen that the distance of the crank-pin _K_ from the center
of the crank disk _N_ will depend entirely upon the stroke of the
engine. It may be well to mention here that the worker should always
start designing his engine by first determining the bore and stroke.
Everything depends upon these two factors. It is also well to mention
here that the piston should never travel completely to the top of the
cylinder--a small space must always be left for the steam to expand.
One eighth of an inch is plenty of space to leave.

It will be noticed that the valve mechanisms on the particular engine
shown bear no resemblance to the _D_ valve previously described. The
holes _G_ which are bored around the cylinder are the exhaust ports. It
will be seen that when the piston is at the end of its downward stroke
it uncovers these exhaust ports and permits the steam to escape. The
momentum of the flywheel _A_ pushes the piston upward, closing these
holes. As these holes are closed the valve _H_ uncovers the entrance _I_
and permits steam to enter from the boiler through _J_. By the time the
piston has reached the upward limit of its stroke a considerable steam
pressure has developed on top of the cylinder, and this again forces the
piston downward. Thus the same cycle of movement is gone through
repeatedly.

The valve on this little engine is extremely simple. It consists of a
circular piece of brass drilled out, as shown. A hole (_I_ and _J_) is
drilled transversely through this. The little cylinder shown in the
insert at _O_ slides in the larger hole, and when it is at its upper
limit it cuts off the steam. At the proper intervals the valve is pulled
down by the eccentric _C_. It will be seen that the moving parts, i.e.,
the valve and the piston, must be properly timed. That is, the eccentric
_C_ must be mounted on the crank-shaft _B_ so that the valve will close
and open at proper intervals. When the engine is made, the eccentric can
be shifted about by means of a set-screw, _Q_, until the engine operates
satisfactorily. This set-screw is used to hold the eccentric to the
crank-shaft. The word eccentric merely means "off center." Thus the
eccentric in this case is formed by