Half of the wear
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.
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