TM 5-3825-270-23&P
0218
SCOPE - Continued
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 Course-UNC) and fine (Unified Fine-UNF). These groups are
defined by the number of threads per inch in the bolt shanks. In addition, threads are categorized by thread class,
which is a measure of the degree between threads of the bolt or screws (external threads) and threads of attaching
nut or tapped hole (internal threads of 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.
INTERNAL
INTERNAL
EXTERNAL
1A
1B
LOOSE FIT
2A
2B
MEDIUM FIT
3A
3B
CLOSE FIT
Figure 3. Fastener Size.
NOTE
Unless followed with "LH" (e.g., 3/4-10 UNC-2A-LH), threads are right-hand.
Figure 4. Thread Pattern.
