Figure 2-21.Bolt heads.
head-hex bolt has three holes drilled in the head,
connecting opposite sides of the hex.
Seven additional types of bolt heads are shown in
figure 2-21. Notice that view A shows an eyebolt,
often used in flight control systems. View B shows a
countersunk-head, close-tolerance bolt. View C
shows an internal-wrenching bolt. Both the
countersunk-head bolt and the internal-wrenching
bolt have hexagonal recesses (six-sided holes) in their
heads. They are tightened and loosened by use of
appropriate sized Allen wrenches. View D shows a
clevis bolt with its characteristic round head. This
head may be slotted, as shown, to receive a common
screwdriver or recessed to receive a Reed-and-Prince
or a Phillips screwdriver.
View E shows a torque-set wrenching recess that
has four driving wings, each one offset from the one
opposite it. There is no taper in the walls of the
recess. This permits higher torque to be applied with
less tendency for the driver to slip or cam out of the
View F shows an external-wrenching head that
has a washer face under the head to provide an
increased bearing surface. The 12-point head gives a
greater wrench gripping surface.
View G shows a hi-torque style driving slot. This
single slot is narrower at the center than at the outer
portions. This and the center dimple provide the slot
with a bow tie appearance. The recess is also
undercut in a taper from the center to the outer ends,
producing an inverted keystone shape. These bolts
must be installed with a special hi-torque driver
adapter. They must also be driven with some type of
torque-limiting or torque-measuring device. Each
diameter of bolt requires the proper size of driver for
that particular bolt.
The bolts are available in
standard and reduced 100-degree flush heads. The
reduced head requires a driver one size smaller than
the standard head.
BOLT THREADS.Another structural feature
in which bolts may differ is threads. These usually
come in one of two types: coarse and fine. The two
are not interchangeable. For any given size of bolt