e figs. 29 and 31), that is, the
roller should not bear against the cam, except during that portion of
the stroke in which it is actually operating the valve, viz., from F to
E (fig. 31). A small stop interposed between the lever and some
convenient part of the engine, such as the side-shaft bracket bearing,
answers this purpose.

[Illustration: FIG. 33.]

[Illustration: FIG. 34.]

The size and shape of the exhaust cam is found in the same manner as
above described; the angle through which it operates is greater than
that of the air cam, and is shown in fig. 33. A fair margin should be
allowed for filing or machining these castings up; the shape and sizes
arrived at by the above described method being finished measurements.
Fig. 34 gives the outline of an exhaust cam worked out from the setting
diagram, fig. 33.

[Illustration: FIG. 35.]

[Illustration: FIG. 37.]

[Illustration: FIG. 41.]

[Illustration: FIG. 42.]

We may now consider the relative positions these two cams will occupy
when keyed up on the side shaft. Assuming that we have both cams
finished to the proper shape and size, and the keyway cut in the side
shaft, we can commence to mark off the position of keyway in the air
cam. With the crank in the position shown in fig. 35, the air cam is
slipped on to the side shaft and brought to the position shown in fig.
32. The keyway being already cut in the side shaft, the position for
that in the cam may be scribed off, as shown by dotted lines (fig. 32),
the cam removed, and the keyway cut. It is as well, however, to check
this mark by turning the crank round to position shown in fig. 37,
_i.e._, the closing of air valve. The side shaft will also turn through
exactly half this angle, so that when the cam is again slipped on the
latter, the scriber marks and keyway in shaft should be exactly in line,
as they were in fig. 32, and the fall of the cam--the closing
portion--should just be touching roller R, but not sufficient to keep
the valve open (see fig. 38). The slightest movement of the crank from
this point in a forward direction should result in a little play being
felt in the lever L, assuming that the cam is also moved just enough to
keep the scriber marks in line with the existing keyway.

[Illustration: FIG. 36.]

[Illustration: FIG. 38.]

[Illustration: FIG. 39.]

By these operations it will be at once evident whether the cam is too
large or too small. Supposing it is too small, we will obt