The I.D. of NPT pipe, nipples, couplings, bards, etc. is not specified -It depends on the product's intended usage or how heavy the pipe's wall needs to be. All NPT fittings have a maximum male O.D. , a certain thread pitch, and a recommended thread length for proper engagement. There is a taper of ~ 3.1/2 Deg. (3/4" per foot or 3.577 Deg.) on the male and female threads that causes a wedge-like engagement of the threads.
Teflon tape is not a sealant -it is a lubricant.
It reduces friction in the joint and allows the pipe to be tightened further where the interference fit creates the seal.
Properly made pipe fittings should turn 2-3 or 2.1/2 - 3.1/2 turns by hand -depending on who you ask / intended usage, materials, etc.