I am a design engineer for the Navy, please don't hold that against me, and I am currently working on a project for the Marines to defeat IEDs in Afghanistan. I need to know at what point an external thread engages with an internal thread in order for holes on one threaded part to line up with holes on the other threaded part. I figure that happens when the height of the internal and external threads are half the specified thread height. Does that make sense? Trouble is, I don't know where that occurs. The thread height is zero at some point and over some angle finally reaches the full height. I have never seen anything about this but I figure somebody knows something because a tap or die has that designed into it. Can anybody shed some light on this? Thanks.