Which thread specification is given for steel in the example?

• A. 1/4-20 UNC threads in aluminum
• B. 1/4-28 UNF threads in titanium
• C. 1/4-28 UNF threads in aluminum
• D. 1/4-28 UNF threads in steel

Answer: (D)

Thread specifications specify diameter, pitch, and form, and the material context guides which pairing is shown. In the example, the material is steel, so the description uses a fine thread form (UNF) with a 1/4 inch diameter and 28 threads per inch. The fine UNF form provides more thread engagement in a steel joint, which helps with alignment and load distribution in rigid metal assemblies. That’s why 1/4-28 UNF threads in steel is the appropriate description.

The other options mix different materials or use a coarse thread form, which wouldn’t match the steel example (for instance, coarse UNC is common with softer materials like aluminum). So the combination that fits both the material (steel) and the thread form (UNF) is the one specified.