Figure 2-18.Heli-coil insert.
problem by using various types of screws, bolts, and
Bolts and screws are similar in that both have a
head at one end and a screw thread at the other, but
there are several differences between them. The
threaded end of a bolt is always relatively blunt, while
that of a screw may be either blunt or pointed. The
threaded end of a bolt must be screwed into a nut, but
the threaded end of the screw may fit into a nut or
other female arrangement, or directly into the material
A bolt has a fairly short threaded
section and a comparatively long grip length (the
unthreaded part); a screw may have a longer threaded
section and no clearly defined grip length. A bolt
assembly is generally tightened by turning its nuts.
Its head may or may not be designed to be turned. A
screw is always designed to be turned by its head.
Another minor but frequent difference between a
screw and a bolt is that a screw is usually made of
lower strength materials.
Threads on aircraft bolts and screws are of the
American National Standard type. This standard
contains two series of threads: national coarse (NC)
and national fine (NF) series. Most aircraft threads
are of the NF series.
Threads are also produced in right-hand and
A right-hand thread advances into
engagement when turned clockwise.
thread advances into engagement when turned
Threads are sized by both the diameter and the
number of threads per inch.
The diameter is
designated by screw gauge number for sizes up to 1/4
inch, and by nominal size for those 1/4 inch and
larger. Screw gauge numbers range from 0 to 12,
except that numbers 7, 9, and 11 are omitted. Threads
are designated by the diameter, number of threads per
inch, thread series, and class in parts catalogs, on
blueprints, and on repair diagrams.
For example, No. 8-32NF-3 indicates a No. 8 size
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.
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 2-19 shows
both types of information.
Figure 2-19.Bolt terms and dimensions.