H-4. TIGHTENING METAL FASTENERS (continued).
H-5. 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
(Table H-4), which is a measure of the degree between threads of bolt or screw (external threads) and threads of the
attaching nut or tapped hole (internal threads) of the attaching nut or tapped hole (internal threads). The most common
thread class for bolts and screws is Class 2.