Figure 2-20.-Correct and incorrect grip lengths.

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 slots.

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.