A tap drill size is theoretically equal to which dimension?

A tap drill size is theoretically equal to which dimension?

• A. The minor diameter of the thread
• B. The major diameter of the thread
• C. The pitch diameter of the thread
• D. The thread length

Answer: (A)

The hole you drill to create an internal thread is sized to the thread’s root, which is the smallest diameter in the thread profile. That root diameter is the minor diameter. When you cut threads with a tap, material is removed from the sides of the hole to form the thread, so the drilled hole ends up matching the minor diameter as the base of the thread. The major diameter is the outer limit of the thread, and the pitch diameter is the midline where the thread’s thickness is measured—these aren’t the sizes used to select the tap drill. The thread length doesn’t determine the pre-drill size either. So the tap drill size is theoretically equal to the minor diameter of the thread.

The hole you drill to create an internal thread is sized to the thread’s root, which is the smallest diameter in the thread profile. That root diameter is the minor diameter. When you cut threads with a tap, material is removed from the sides of the hole to form the thread, so the drilled hole ends up matching the minor diameter as the base of the thread. The major diameter is the outer limit of the thread, and the pitch diameter is the midline where the thread’s thickness is measured—these aren’t the sizes used to select the tap drill. The thread length doesn’t determine the pre-drill size either. So the tap drill size is theoretically equal to the minor diameter of the thread.