centre, where the exhaust valve should close, and
so on till the opening and closing of each valve has been checked. It
will be noticed that the air, and sometimes the gas, valve opens before
the exhaust closes. This overlap is necessary; and it will be found that
the smaller the engine and the higher the speed the greater this overlap
will be to obtain good results, although a good deal of individual
judgment must be used in settling the exact amount of overlap, as the
requisite amount may, to get the best results, vary in different engines
of precisely the same dimensions and type.

When dealing with engines which have no separate gas valve--the gas
being admitted with the air, which is sometimes the case with very small
engines--the above notes referring to the gas setting independently,
will, of course, not hold good.

It may be mentioned with regard to the lump on the opening side of the
exhaust cam, that this if overdone is found to be detrimental on large
engines, and even on small ones. If it is too large, it will cause both
exhaust valve and seat to become burnt and pitted, due to the surface
being exposed to the exceedingly high temperature of the expanding
gases. If it is too large, it is equivalent to opening the exhaust valve
too early, and the effect is the same, viz., a waste of power and damage
to the valve and its seat.

[Illustration: FIG. 43.]

[Illustration: FIG. 44.--Brake Testing.]

The method of grinding in the valves to their seats with emery powder
and oil is so well known that no further description is needed here. We
give, however, in fig. 43 a sketch showing a very expeditious way of
dealing with very badly worn or burnt seats. The sketch explains itself.
Such a tool is readily made; even the cutter could be turned and filed
up to shape and then hardened at home. By lightly tapping in the taper
cotter pin little by little, sufficient pressure is put on the cutter to
make it an easy matter to completely re-face an old seat or form a new
one. A T-wrench or "tommy" can be used to work the cutter spindle. The
lower part of the latter must be the same diameter as the existing valve
spindle; the bush acts as a guide; and as the bevel of the cutter
should be the same as that of the valve, a very little grinding in with
emery powder is required to finish the job off.

In fig. 44 we give a diagram showing the method of testing for Brake
H.P. of engine, as it is frequently interesting to make suc