designed to be turned. A screw is always designed
the nut at least two full threads. If the bolt is too
to be turned by its head. Another minor difference
short, it will not extend out of the bolt hole far
between a screw and a bolt is that a screw is
enough for the nut to be securely fastened. If it
usually made of lower strength materials.
is too long, it may extend so far that it interferes
with the movement of nearby parts.
Threads on aircraft bolts and screws are of the
American National Aircraft Standard type, This
In addition, if a bolt is too long or too short,
standard contains two series of threads--national
its grip will usually be the wrong length. As
coarse (NC) and national fine (NF). Most aircraft
shown in figure 2-16, the grip length should be
threads are of the NF series.
approximately the same as the thickness of the
material to be fastened. If the grip is too short,
Bolts and screws may have right- or left-hand
threads. A right-hand thread advances into
the threads of the bolt will extend into the bolt
engagement when turned clockwise. A left-hand
hole. The bolt may act like a reamer when the
material is vibrating. If the grip is too long, the
thread advances into engagement when turned
nut will run out of threads before it can be
counterclockwise.
tightened. In this event, a bolt with a shorter grip
Bolts
should be used. If the bolt grip extends only a
short distance through the hole, a washer maybe
Many types of bolts are used in modern air-
used.
craft, and each type is used to fasten something
A second bolt dimension that must be con-
in place. Before we discuss some of these types,
sidered is diameter. As shown in figure 2-15, the
it might be helpful to list and explain some
diameter of the bolt is the thickness of its shaft.
commonly used bolt terms. You should know the
The results of using a wrong diameter bolt
names of bolt parts and be aware of the bolt
should be obvious. If the bolt is too big, it
dimensions that must be considered in selecting
cannot enter the bolt hole. If the diameter is too
a bolt.
small, the bolt has too much play in the bolt hole.
The three principal parts of a bolt are the
The third and fourth bolt dimensions that
head, grip, and threads, as shown in figure 2-15.
should be considered when you choose a bolt
Two of these parts might be well known to you,
but perhaps grip is an unfamiliar term. The grip
is the unthreaded part of the bolt shaft. It extends
from the threads to the bottom of the bolt head.
The head is the larger diameter of the bolt and
may be one of many shapes or designs.
To choose the correct replacement for an
unserviceable bolt, you must consider the length
of the bolt. As shown in figure 2-15, the bolt
length is the distance from the tip of the threaded
end to the head of the bolt. Correct length selec-
tion is indicated when the bolt extends through
Figure 2-15 .-Bolt terms and dimensions.
Figure 2-16.-Correct and incorrect grip lengths.
2-13
