FAA Advisory Circular 43.13-1B
Acceptable Methods, Techniques, and Practices
Aircraft Inspection and Repair
AC 43.13-1B | 7. Aircraft Hardware, Control Cables, and Turnbuckles | 3. Bolts | 7-36. Identification
9/8/98
SECTION 3. BOLTS
AC 43.13-1B
7-34. GENERAL. “Hardware” is the term
used to describe the various types of fasteners
and small items used to assemble and repair
aircraft structures and components. Only
hardware with traceability to an approved
manufacturing process or source should be
used. This traceability will ensure that the
hardware is at least equal to the original or
properly-altered condition. Hardware that is
not traceable or is improperly altered, may be
substandard or counterfeit, since their physical
properties cannot be substantiated. Selection
and use of fasteners are as varied as the types
of aircraft; therefore, care should be taken to
ensure fasteners are approved by the Federal
Aviation Administration (FAA) for the in-
tended installation, repair, or replacement.
Threaded fasteners (bolts/screws) and rivets
are the most commonly used fasteners because
they are designed to carry shear and/or tensile
loads.
7-35. BOLTS. Most bolts used in aircraft
structures are either general-purpose, internal-
wrenching, or close-tolerance AN, NAS, or
MS bolts. In certain cases, fastener manufac-
turers produce bolts of different dimensions or
greater strength than the standard types. Such
bolts are made for a particular application,
and it is of extreme importance to use like
bolts in replacement. Design specifications
are available in MIL-HDBK-5 or USAF/Navy
T.O. 1-1A-8/NAVAIR 01-1A-8. References
should be made to military specifications and
industry design standards such as NAS, the
Society of Automotive Engineers (SAE), and
Aerospace Material Standards (AMS). Typical
bolt types are shown in table 7-12.
7-36. IDENTIFICATION. Aircraft bolts
may be identified by code markings on the bolt
heads. These markings generally denote the
material of which the bolt is made, whether the
bolt is a standard AN-type or a special-purpose
bolt, and sometimes include the manufacturer.
a. AN standard steel bolts are marked
with either a raised dash or asterisk, corrosion-
resistant steel is marked by a single dash, and
AN aluminum-alloy bolts are marked with two
raised dashes.
b. Special-purpose bolts include high-
strength, low-strength, and close-tolerance
types. These bolts are normally inspected by
magnetic particle inspection methods. Typical
markings include “SPEC” (usually heat-treated
for strength and durability), and an aircraft
manufacturer’s part number stamped on the
head. Bolts with no markings are low strength.
Close-tolerance NAS bolts are marked with
either a raised or recessed triangle. The mate-
rial markings for NAS bolts are the same as for
AN bolts, except they may be either raised or
recessed. Bolts requiring non-destructive in-
spection (NDI) by magnetic particle inspection
are identified by means of colored lacquer, or
head markings of a distinctive type. (See fig-
ure 7-1.)
7-37. GRIP LENGTH. In general, bolt grip
lengths of a fastener is the thickness of the
material the fastener is designed to hold when
two or more parts are being assembled. Bolts
of slightly greater grip length may be used,
provided washers are placed under the nut or
bolthead. The maximum combined height of
washers that should be used is 1/8 inch. This
limits the use of washers necessary to compen-
sate for grip, up to the next standard grip size.
Over the years, some fasteners specifications
have been changed. For this reason, it is rec-
ommended when making repairs to an aircraft,
whose original hardware is being replaced, that
you must first measure the bolt before order-
ing, rather than relying on the parts manual for
Par 7-34
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