The tap drill for a 3/8-16 UNC internal thread is how many inches in diameter?

To find the correct tap drill size for a 3/8-16 UNC internal thread, it is essential to understand the relationship between the diameter of the internal thread and the appropriate tap drill size.

For a 3/8-16 UNC thread, the nominal size is 3/8 inches and the thread pitch is 16 threads per inch. The appropriate tap drill size can be calculated using the formula for determining the diameter of the tap drill based on the thread size and pitch. A common guide suggests that the tap drill diameter for a given thread size is typically close to the major diameter of the bolt minus the thread pitch.

In this case, the major diameter of the 3/8-16 UNC thread is 0.375 inches (which is 3/8 inches). The general recommendation is to subtract the pitch from this diameter. The pitch for a 16-thread per inch classification is approximately 0.0625 inches (1 divided by the number of threads per inch).

Therefore, the calculation would be:

0.375 inches (major diameter) - 0.0625 inches (pitch) = 0.3125 inches.

This gives us the appropriate tap drill size of 0.3125 inches