Which statement best describes UNS (Specified United National Thread Series) threads?

• A. They require a specific threads-per-inch (TPI) for a given diameter.
• B. They have a universal TPI for all diameters.
• C. They are only used for internal threads.
• D. They are restricted to metric diameters.

Answer: (A)

UNS threads are a set of Unified National Special threads defined for exact diameters with a fixed threads-per-inch. For a given diameter within the UNS family, the TPI is specified and must be used, because these sizes aren’t covered by the standard coarse or fine series. This is why the statement that they require a specific TPI for a given diameter is the best description.

The other aspects don’t fit: there isn’t a universal TPI shared by all UNS diameters, UNS threads can be external or internal, and UNS is a U.S. imperial standard rather than a metric system.