What is the correct tap drill for a 3/4-14 NPT pipe thread?

For a 3/4-14 NPT (National Pipe Tapered) pipe thread, the correct tap drill size is indeed 59/64 inch. This size is specifically designed to create the appropriate diameter hole that ensures an optimal fit for the threads when tapping. The 3/4-14 designation indicates that the outer diameter of the pipe thread is approximately 3/4 inch, and the "14" refers to the number of threads per inch.

Using the correct tap drill size is crucial for achieving the desired thread form and ensuring a tight seal when connecting pipes. A hole that is too small will not accommodate the tapering needed for proper threading, while a hole that is too large may not allow for effective sealing or may lead to weak connections.

The other drill sizes would not provide the proper clearance or thread engagement necessary for a 3/4-14 NPT thread, making 59/64 inch the appropriate choice for achieving the correct internal dimensions for consistent and reliable threaded connections.