Inflight mishaps continue to happen at an
micrometer is used for measuring outside dimen-
alarming rate. Many of these mishaps are due to
improper hardware selection and installation. For
sions, such as the diameter of a piece of round
stock. Use the screw thread micrometer to
example, mishaps involving aircraft fires can often
determine the pitch diameter of screws. The
be attributed to the chafing of fluid lines and wire
inside micrometer is used for measuring inside
bundles, caused by improperly clamped parts.
the inside diameter of a tube or hole, the bore of
a cylinder, or the width of a recess. Use the depth
In modern aircraft construction, thousands of
micrometer for measuring the depth of holes or
rivets are used. Many parts require frequent
dismantling or replacement. It is more practical
for you to use some form of threaded fastener.
clean and lightly oiled. Make sure they are placed
Some joints require greater strength and rigidity
in a case or box when they are not in use. Anvil
than can be provided by riveting. We use various
faces must be protected from damage and must
types of bolts and nuts to solve this problem.
not be cleaned with emery cloth or other
Bolts and screws are similar in that both have
a head at one end and a screw thread at the other.
However, there are several differences between
them. The threaded end of a bolt is always
relatively blunt. A screw may be either blunt or
pointed. The threaded end of a bolt must be
screwed into a nut. The threaded end of the screw
may fit into a nut or directly into the material
The importance of aircraft hardware is often
overlooked because of the small size of most
being secured. A bolt has a fairly short threaded
items. The safe and efficient operation of any air-
section and a comparatively long grip length (the
craft depends upon the correct selection and use
unthreaded part). A screw may have a longer
threaded section and no clearly defined grip
of aircraft hardware. You must make sure that
length. A bolt assembly is generally tightened by
items of aircraft hardware remain tightly secured
turning a nut. The bolt head mayor may not be
in the aircraft.
Figure 2-14.-Common types of micrometers.