bolt and may be one of many shapes or designs. The
frequent difference between a screw and a bolt is that a
head keeps the bolt in place in one direction, and the nut
screw is usually made of lower strength materials.
used on the threads keeps it in place in the other
Threads on aircraft bolts and screws are of the
direction.
American National Standard type. This standard
To choose the correct replacement, several bolt
contains two series of threads: national coarse (NC) and
dimensions must be considered. One is the length of the
national fine (NF) series. Most aircraft threads are of
bolt. Note in figure 3-10 that the bolt length is the
the NF series.
distance from the tip of the threaded end to the head of
Threads are also produced in right-hand and
the bolt. Correct length selection is indicated when the
left-hand types. A right-hand thread advances into en-
chosen bolt extends through the nut at least two full
gagement when turned clockwise. A left-hand thread
threads. In the case of flat-end bolts or chamfered
advances into engagement when turned counter-
(rounded) end bolts, at least the full chamfer plus one
clockwise.
full thread should extend through the nut. See figure
Threads are sized by both the diameter and the
3-10. If the bolt is too short, it may not extend out of the
number of threads per inch. The diameter is designated
bolt hole far enough for the nut to be securely fastened.
by screw gauge number for sizes up to 1/4 inch, and by
If it is too long, it may extend so far that it interferes
nominal size for those 1/4 inch and larger. Screw gauge
with the movement of nearby parts. Unnecessarily long
numbers range from 0 to 12, except that numbers 7, 9,
bolts can affect weight and balance and reduce the
and 11 are omitted. Threads are designated by the
aircraft payload capacity.
diameter, number of threads per inch, thread series, and
In addition, if a bolt is too long or too short, its grip
class in parts catalogs, on blueprints, and on repair
is usually the wrong length. As shown in figure 3-11,
diagrams.
grip length should be approximately the same as the
For example, No. 8-32NF-3 indicates a No. 8 size
thickness of the material to be fastened. If the grip is too
thread, 32 threads per inch, national fine series, and a
class 3 thread. Also, 1/4-20NC-3 indicates a 1/4-inch
thread, 20 threads per inch, national coarse series, and a
class 3 thread. A left-hand thread is indicated by the
letters LH following the class of thread.
Bolts
Many types of bolts are used on aircraft. However,
before discussing some of these types, it might be
helpful to list and explain some commonly used bolt
terms. You should know the names of bolt parts and be
aware of the bolt dimensions that must be considered in
selecting a bolt. Figure 3-10 shows both types of
information.
The three principal parts of a bolt are the head,
thread, and grip. The head is the larger diameter of the
Figure 3-10.--Bolt terms and dimensions.
Figure 3-11.--Correct and incorrect grip lengths.
3-6
