INSTALLATION AND TORQUING
Matching Nuts. Matching nuts require a minimum height equal to the basic diameter of the bolt. The same
is true of tapped holes. In tapped softer materials, the depth of the tapped hole should be 1.5 times the
basic diameter of the bolt.
Threaded Protrusion. In all installations, bolts, studs, and screws must extend through nut at least a length
equivalent to two complete threads. This applies to both self-locking and plain nuts.
Torquing Self-Locking Nuts. To obtain the correct recommended torque value on self-locking nuts, nut
must be tightened until it is one turn from the beginning of seating. At this point, if torque is less than 0.33 of
recommended torque, it should be disregarded and the nut tightened to recommended torque value. If
torque is 0.33 or more of the recommended torque, it should be added to recommended torque. Example:
Recommended torque is 50-70 Ib in. (6-8 N·m). The torque at one turn from seating is 30 lb in. (3 N·m). The
correct torque wrench reading would be 80-100 Ib in. (9-11 N·m).
Re-torquing Fasteners. Procedures intended for installing metal fasteners can cause incorrect readings
when used to check or re-torque already installed fasteners during maintenance. Before checking or re-
torquing an already installed threaded fastener, first mark the fastener and its companion components so
the marks are in line. Second, back it off a 0.25 turn to loosen it. Torque it to specification with an even
steady pull on torque wrench. The marks should be in line; if not, the marks will indicate fastener was under
or over torqued.
Standard Torque Charts. Standard torque charts have been established for dry and wet torque conditions.
Surface variations such as thread roughness, scale paint, lubrication (oil, grease, etc.), hardening, and
plating may alter these values considerably. Tables 2 and 3 are standard torque charts.
To find the grade of the screw that is to be installed, match the markings on the head to the correct picture
of CAPSCREW HEAD MARKINGS in Tables 2 and 3. Manufacturer s marks may vary.