Figure 2-22.—Bolt head markings.
there is a different number of coarse and fine threads
per inch. For instance, consider the 1/4-inch bolts.
Some are called 1/4-28 bolts because they have 28
fine threads 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 MATERIAL.—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
head markings that help to identify the material from
which they are manufactured. Figure 2-22 shows the
tops of several hex-head bolts, each marked to
indicate the type of bolt material.
BOLT IDENTIFICATION.—Unless current
directives specify otherwise, every unserviceable bolt
should be replaced with a bolt of the same type. Of
course, substitute and interchangeable items are
sometimes available, but the ideal fix is a bolt-for-bolt
replacement. The part number of a needed bolt may
be obtained by referring to the illustrated parts
breakdown (IPB) for the aircraft concerned. Exactly
what this part number means depends upon whether
the bolt is AN (Air Force-Navy), NAS (National
Aircraft Standard), or MS (Military Standard).
AN Part Number.—There are several classes of
AN bolts, and in some instances their part numbers
reveal slightly different types of information.
However, most AN numbers contain the same type of
information.
Figure 2-23 shows a breakdown of a typical AN
bolt part number.
Like the AN rivets discussed
earlier, it starts with the letters AN. Next, notice that
a number follows the letters. This number usually
consists of two digits. The first digit (or absence of it)
shows the class of the bolt. For instance, in figure
2-23, the series number has only one digit, and the
absence of one digit shows that this part number
represents a general-purpose hex-head bolt.
However, the part numbers for some bolts of this class
have two digits.
In fact, general-purpose hex-head
bolts include all part numbers beginning with AN3,
AN4, and so on, through AN20. Other series numbers
and the classes of bolts that they represent are as
follows:
AN21 through AN36—clevis bolts
AN42 through AN49—eyebolts
The series number shows another type of
information other than bolt class. With a few
exceptions, it indicates bolt diameter in sixteenths of
Figure 2-23.—AN bolt part number breakdown.
2-18
