The  elastic  stop  nut  is  constructed  with  a nonmetallic (nylon) insert, which is designed to lock the nut in place. The insert is unthreaded and has a smaller diameter than the inside diameter of the nut. Self-locking nuts are generally suitable for reuse in noncritical applications provided the threads have not been damaged. If the locking material has not been damaged or permanently distorted, it can be reused. NOTE:   If any doubt exists about the condition of a nut, use a new one! When you anchor lightweight parts, the sheet spring nut may be used. See figure 5-8. Applications include supporting line clamps, electrical equipment, and small access doors. It is made of sheet spring steel and is cut so as to have two flaps. The ends of these flaps are notched to form a hole that is somewhat smaller in diameter than the screw used. The sheet spring nut has a definite arch that tends to flatten out as the screw pulls the flaps in toward the threads. This flattening action forces the flaps of the nut tightly into the threads of the screw. The springiness of the sheet spring nut pushes upward on the screw threads, binding them and locking the screw in place. With the sheet spring nut, either a standard or a sheet metal self-tapping screw is used. INSTALLATION OF NUTS AND BOLTS You must be certain that each bolt is made of the correct material. Examine the markings on the head to determine whether a bolt is steel or aluminum alloy. It is of extreme importance to use like bolts in replacement. In every case, refer to the applicable maintenance instruction manual and illustrated parts breakdown. Be sure that washers are used under the heads of both bolts and nuts unless their omission is specified. A washer guards against mechanical damage to the material being bolted and prevents corrosion of the structural members. An aluminum alloy washer may be used under the head and nut of a steel bolt securing aluminum alloy or magnesium alloy members. Corrosion will attack the washer rather than the members. Steel washers should be used when joining steel members with steel bolts. Whenever possible, the bolt should be placed with the head on top or in the forward position. This positioning helps prevent the bolt from slipping out if the nut is accidentally lost. Make sure that the bolt grip length is correct. Generally speaking, the grip length should equal the thickness of the material being bolted together. Not more than one thread should bear on the material, and the shank should not protrude too far through the nut. Figure 5-2 shows examples of correct and incorrect grip length. Application of Torque Torque is the amount of twisting force applied when you are tightening a nut. If torque values are specified in the appropriate manual, a torque wrench must be used. Regardless of whether torque values are specified or not, all nuts in a particular installation must be tightened a like amount. This permits each bolt in a group to carry its share of the load. It is a good practice to use a torque wrench in all applications. Safetying of Nuts and Bolts It is very important that all nuts except the self-locking type be safetied after installation. This prevents nuts from loosening in flight because of vibration. Methods of safetying are discussed later in this chapter. Q5-2. What are the three principal parts of a bolt? Q5-3. What are the three most commonly used screws in aircraft construction? Q5-4. What general group of aircraft nuts require an external locking device, such as cotter pins, safety wire, or locknuts? 5-6 ANF0508 TOP VIEW SIDE VIEW STARTING POSITION DOUBLE-LOCKED POSITION INWARD THREAD LOCK A B C D Figure 5-8.—Sheet spring nut.