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Figure 2-22.—Bolt head markings. there is a different number of coarse and fine threads per inch.    For instance, consider the 1/4-inch bolts. Some are called 1/4-28 bolts because they have 28 fine threads per inch. Others have only 20 coarse threads per inch and are called 1/4-20 bolts. To force one size of threads into another size, even though both are 1/4 of an inch, can strip the finer threads or softer metal. The same thing is true concerning the other sizes of bolts; therefore, make certain that bolts you select have the correct type of threads. BOLT MATERIAL.—The type of metal used in an aircraft bolt helps to determine its strength and its resistance to corrosion. Therefore, make certain that material is considered in the selection of replacement bolts. Like solid shank rivets, bolts have distinctive head markings that help to identify the material from which they are manufactured. Figure 2-22 shows the tops of several hex-head bolts, each marked to indicate the type of bolt material. BOLT IDENTIFICATION.—Unless current directives specify otherwise, every unserviceable bolt should be replaced with a bolt of the same type. Of course, substitute and interchangeable items are sometimes available, but the ideal fix is a bolt-for-bolt replacement. The part number of a needed bolt may be obtained by referring to the illustrated parts breakdown (IPB) for the aircraft concerned. Exactly what this part number means depends upon whether the bolt is AN (Air Force-Navy), NAS (National Aircraft Standard), or MS (Military Standard). AN Part Number.—There are several classes of AN bolts, and in some instances their part numbers reveal slightly different types of information. However, most AN numbers contain the same type of information. Figure 2-23 shows a breakdown of a typical AN bolt part number. Like the AN rivets discussed earlier, it starts with the letters AN. Next, notice that a number follows the letters. This number usually consists of two digits. The first digit (or absence of it) shows the class of the bolt. For instance, in figure 2-23, the series number has only one digit, and the absence of one digit shows that this part number represents a general-purpose hex-head bolt. However, the part numbers for some bolts of this class have two digits. In fact, general-purpose hex-head bolts include all part numbers beginning with AN3, AN4, and so on, through AN20. Other series numbers and the classes of bolts that they represent are as follows: AN21 through AN36—clevis bolts AN42 through AN49—eyebolts The series number shows another type of information other than bolt class. With a few exceptions, it indicates bolt diameter in sixteenths of Figure 2-23.—AN bolt part number breakdown. 2-18



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