Fastener Size and Thread Pattern
Threaded fasteners are categorized according to the dia. 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 1 in. (25.4 mm) on the bolt shank. In addition, threads are categorized by thread class (see Table
4), 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 4. Thread Classes and Description
Thread patterns are as follows:
Unless followed with -LH (e.g., 3/4-10-UNC-2A-LH), threads are right-hand.