Figure 3-13.--Bolt head markings.
which they are manufactured. Figure 3-13 shows the
This permits higher torque to be applied with less
tops of several hex-head bolts, each marked to indicate
tendency for the driver to slip or cam out of the slots.
the type of bolt material.
View F shows an external-wrenching head that has
a washer face under the head to provide an increased
BOLT IDENTIFICATION.--Unless current
bearing surface. The 12-point head gives a greater
directives specify otherwise, every unserviceable bolt
wrench gripping surface.
should be replaced with a bolt of the same type. Of
course, substitute and interchangeable items are
View G shows a hi-torque style driving slot. This
sometimes available, but the ideal fix is a bolt-for-bolt
single slot is narrower at the center than at the outer
replacement. The part number of a needed bolt may be
portions. This and the center dimple provide the slot
obtained by referring to the illustrated parts breakdown
with a bow tie appearance. The recess is also undercut
(IPB) for the aircraft concerned. Exactly what this part
in a taper from the center to the outer ends, producing
number means depends upon whether the bolt is AN
an inverted keystone shape. These bolts must be
(Air Force-Navy), NAS (National Aircraft Standard),
installed with a special hi-torque driver adapter. They
or MS (Military Standard).
must also be driven with some type of torque-limiting
or torque-measuring device. Each diameter of bolt
AN Part Numbers.--There are several classes of
requires the proper size of driver for that particular bolt.
AN bolts, and in some instances their part numbers
The bolts are available in standard and reduced
reveal slightly different types of information. However,
100-degree flush heads. The reduced head requires a
most AN numbers contain the same type of
driver one size smaller than the standard head.
information.
BOLT THREADS.--Another structural feature in
Figure 3-14 shows a breakdown of a typical AN
which bolts may differ is threads. These usually come
bolt part number. Like the AN rivets discussed earlier, it
in one of two types: coarse and fine. The two are not
starts with the letters AN. Next, notice that a number
interchangeable. For any given size of bolt there is a
follows the letters. This number usually consists of two
different number of coarse and fine threads per inch.
digits. The first digit (or absence of it) shows the class
For instance, consider the 1/4-inch bolts. Some are
of the bolt. For instance, in figure 3-14, the series
called 1/4-28 bolts because they have 28 fine threads
number has only one digit, and the absence of one digit
per inch. Others have only 20 coarse threads per inch
and are called 1/4-20 bolts. To force one size of threads
into another size, even though both are 1/4 of an inch,
can strip the finer threads or softer metal. The same
thing is true concerning the other sizes of bolts;
therefore, make certain that bolts you select have the
correct type of threads.
BOLT MATERIALS.--The type of metal used in
an aircraft bolt helps to determine its strength and its
resistance to corrosion. Therefore, make certain that
material is considered in the selection of replacement
bolts. Like solid shank rivets, bolts have distinctive
Figure 3-14.--AN bolt part number breakdown.
head markings that help to identify the material from
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