The crank case of a motor serves as a foundation for the engine,
furnishes a support for the main bearings in which the crank shaft
revolves and encloses the working parts in such a way as to provide for
their lubrication and protect them from the dust and other substances
which might materially hinder the proper performance of their
functions. To a certain extent the crank case might be compared to the
framework of the grindstone, although the latter does not answer as
many purposes as the real crank case of the motor does.
[Illustration: Fig. 20--The two halves of a Four-cylinder Crank Case.]
The case itself is made of iron or aluminum, and is so put together
that, although practically air-tight, there is still a means provided
for getting inside of it for examination of the working parts or an
adjustment of the bearings.