comes off of each half, and the half next to the key is brought up to
the wrist pin because of the tapering key, while the outside half
remains in one place. You must therefore place back of this half a thin
piece of sheet copper, or a piece of tin will do. Now suppose our boxes
had one-eighth of an inch for wear. When we have taken up this much we
must put in one-sixteenth of an inch backing (as it is called), for we
have reduced the outside half by just that amount. We have also reduced
the front half the same, but as we have said, the tapering key brings
this half up to its place.

Now we think we have made this clear enough and we will leave this and
go back to the key again. You must remember that we stated that the key
was tapering or a wedged shape, and as a wedge, is equally as powerful
as a screw, and you must bear in mind that a slight tap will bring these
two boxes up tight against the wrist pin. Young engineers experience
more trouble with this box than with any other part of the engine, and
all because they do not know how to manage it. You should be very
careful not to get your box too tight, and don't imagine that every time
there is a little knock about your engine that you can stop it by
driving the key down a little more. This is a great mistake that many,
and even old engineers make. I at one time seen a wrist pin and boxes
ruined by the engineer trying to stop a knock that came from a loose
fly-wheel. It is a fact, and one that has never been satisfactorily
explained, that a knock coming from almost any part of an engine will
appear to be in the wrist. So bear this in mind and don't allow
yourself to be deceived in this way, and never try to stop a knock until
you have first located the trouble beyond a doubt.

When it becomes necessary to key up your brasses, you will find it a
good safe way to loosen up the set screw which holds the key, then drive