Which UNF thread is provided as an option for steel in the example?

• A. 1/4-28 UNF
• B. 2-64 UNF
• C. 1/4-28 UNC
• D. 1/4-20 UNC

Answer: (A)

The concept here is reading thread designations and recognizing UNF versus UNC and how the numbers encode diameter and pitch. UNF stands for Unified National Fine, which means a finer thread pitch than UNC for the same nominal diameter.

The notation 1/4-28 UNF means a nominal diameter of 1/4 inch with 28 threads per inch, and the designation UNF specifies the fine-thread family. In the steel example, this is the UNF option that matches the quarter-inch size with a fine pitch.

The other options that are UNC (coarser threads) or that use a different diameter (such as a 2-inch size with 64 TPI) do not fit the example’s given quarter-inch UNF thread. A 1/4-28 UNC would be the coarser thread, and 1/4-20 UNC is also UNC, not UNF, so they’re not the correct UNF choice for the same size.