Bolts

Figure 2-18.—Heli-coil insert. problem by using various types of screws, bolts, and nuts. 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 being secured. 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 left-hand types. A right-hand thread advances into engagement when turned clockwise. A left-hand thread advances into engagement when turned counterclockwise. 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. Bolts 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. 2-15