Select the appropriate tap drill size for tapping a 1/4-28 thread in steel.

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

Answer: (D)

The key idea is that when you tap, you drill a hole slightly smaller than the screw’s major diameter so the tap can form the internal threads properly. For a 1/4-28 UNC thread, the standard tapping chart calls for the No. 1 drill, which is about 0.228 inches. This size provides enough material for the 28 threads per inch to cut cleanly without overloading the tap. If the hole is larger, you’ll end up with too shallow or loose threads; if it’s smaller, the tap has to cut more material, increasing heat, wear, and the risk of breaking the tap or producing weak threads. In steel, using the correct drill size is still essential for good thread form, lubrication, and tool life. So the No. 1 drill (0.228") is the appropriate tap drill for 1/4-28 in steel.