Thread classes define how much (if any) allowance is required between mating threads, and how large the tolerance zone is for the male and female threads.
Since these values are different for internal and external threads, each class identifier (1, 2, or #) must include either an A or B to distinguish between external and internal thread limits.
- Class 1A and 1B
- Liberal tolerance and allowance.
- Easy assembly and disassembly
- Work when slightly damaged
- Not used frequently
- Class 2A and 2B
- Common class for general purpose
- Often the class of nuts, bolts and screws
- Class 3A and 3B
- No allowance built in.
- Possible for internal and external to match size exactly.
- Combinations of Class
- Classes can be combined for different design requirement.
- For example, Class 2A external bolt can be used with a 1B, 2B, or 3B nut.