Which thread size is specified for aluminum in the examples?

• A. 1/4-20 UNC threads in aluminum
• B. 4-40 UNC threads in aluminum
• C. 1/2-13 UNC threads in aluminum
• D. 1/4-28 UNF threads in aluminum

Answer: (A)

The thread size chosen for aluminum in the examples uses a coarse UNC thread on a 1/4 inch diameter. Coarse threads like UNC are preferred for softer metals such as aluminum because they are easier to tap and provide better resistance to thread stripping in thin or soft walls. A 1/4-20 UNC combination is a common, readily available standard for lightweight assemblies, matching typical practice in such examples. The other options either use a finer thread (UNF) or different diameters, which don’t align with what the example specifies. So, 1/4-20 UNC threads in aluminum is the one shown.