There's no such thing as 7/8 NPT. You may be confusing it with the OD of the hole. A 1/2" NPT thread has a major (outside) diameter of 0.84" which is pretty close to 7/8" (0.875"). A hole for a 1" NPT tap would be 1-5/32". Pipe threads are designated by the nominal INSIDE diameter of the pipe (1/2" NPT would be roughly 1/2" inside diameter.)
Sorry, it's the plumber coming out in me.
