NPT is National Pipe Thread
Its cut with a taper, that way it pressure fits the connection. 3 threads is about normal it will go in more with a thread sealer paste (don't use Teflon tape please) there is no real lubricating property in tapes
on the 1/4 one it sounds like there are 2 different thread pitches, don't force it, make sure its a 1/4-20 or a AN, a flair or even metric
