The reference cited that describes tapered pipe threads is specific to FIELD MADE threads. It does not apply to our couplings, many conduit bodies, chase nipples, or EMT fittings.
Likewise, even a plumber will have leaks in the best NPT threads if he fails to use pipe dope. There is a slightly different thread -"dry seal"- that is used where it is required to actually seal without the use of any sealant.
