The answer is actually quite simple. Pipe threads are sized differently than standard straight thread. They are based on the basic size at the top of the threaded hole that has been produced by the pipe tap. The Basic size occurs approximately 12 threads for the front of the taper pipe tap. This is commonly called the 12-thread count. You can therefore run any standard projection pipe tap into the drilled hole approximately 12 threads from the front of the tap, and that should be approximately the Basic size for that pipe thread.
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