To cut an internal thread that has a nominal size of 0.875 in and 10 TPI, what is the minor diameter of the hole that must be drilled?

To cut an internal thread that has a nominal size of 0.875 in and 10 TPI, what is the minor diameter of the hole that must be drilled?

• A. 0.775 in
• B. 0.825 in
• C. 0.875 in
• D. 0.900 in

Answer: (A)

When shaping an internal thread, the hole you drill must match the thread’s minor diameter—the smallest diameter of the threaded hole. The major diameter is given as 0.875 in, and the pitch is the distance between thread peaks, which for 10 TPI is 0.1 in. For standard 60° thread forms, the depth that the thread removes from the major diameter is about the pitch in total (roughly half the pitch on each side of the thread). That means the minor diameter is about the major diameter minus the pitch: 0.875 in − 0.100 in = 0.775 in. So the hole to drill is about 0.775 in. The other options don’t fit because they would either equal the major diameter, be too large, or not account for the pitch reduction needed to form the internal threads. In practice you’d confirm with a tap drill chart for the exact class tolerance, but the fundamental idea is drill to the minor diameter, not the major.

When shaping an internal thread, the hole you drill must match the thread’s minor diameter—the smallest diameter of the threaded hole. The major diameter is given as 0.875 in, and the pitch is the distance between thread peaks, which for 10 TPI is 0.1 in. For standard 60° thread forms, the depth that the thread removes from the major diameter is about the pitch in total (roughly half the pitch on each side of the thread). That means the minor diameter is about the major diameter minus the pitch: 0.875 in − 0.100 in = 0.775 in.

So the hole to drill is about 0.775 in. The other options don’t fit because they would either equal the major diameter, be too large, or not account for the pitch reduction needed to form the internal threads. In practice you’d confirm with a tap drill chart for the exact class tolerance, but the fundamental idea is drill to the minor diameter, not the major.