For the 1/4-28 thread in steel, which option is the correct tap drill size from the data provided?

• A. 0.2062 inch
• B. #1 (0.228)
• C. 0.250 inch
• D. 0.213 inch

Answer: (B)

Tapping a hole requires a pilot hole that yields the correct thread form for the screw after full cutting. For a 1/4-28 UNF thread, the standard pilot hole is about 0.226–0.228 inches so the tap can create 28 threads per inch with good engagement without stripping or weakening the wall. The closest and safest match in common data is a 0.228 inch drill size, which corresponds to a standard small drill. Using a noticeably smaller hole would make tapping difficult or produce overly tight threads; using a larger hole would weaken the threaded engagement. Therefore, the 0.228 inch drill size is the best choice.