TB 43-0218
SECTION II. METAL FASTENERS
3. Fastener Size and Thread Pattern. Threaded fasteners are categorized according to diameter of the fastener shank.
Thread styles are divided into broad groups, the two most common being coarse (Unified Coarse-UNC) and fine (Unified
Fine-UNF). These groups are defined by the number of threads per inch on the bolt shanks. In addition, threads are
categorized by thread class, which is a measure of the degree of fit between the threads of the bolt or screw (external
threads) and the threads of the attaching nut or tapped hole (internal threads). The most common thread class for bolts
and screws is Class 2.
TABLE 2-1. THREAD CLASSES
EXTERNAL
INTERNAL
FIT
1A
1B
LOOSE FIT
2A
2B
MEDIUM FIT
3A
3B
CLOSE FIT
Figure 1. Thread Description
2
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