I hear that this type of threading is supposed to fit together pretty firm. I got a thermoswitch and a hose adapter for my efan. I tried testing it out threading them together but about 1/4 of the way down and it seems to be pretty difficult. I didn't try super hard to thread them together.

So I guess my question is how hard is it supposed to be to mate the two?

 

they are tapered threads. they are supposed to get harder to thread in. I also like to put a bit of RTV on the threads to improve sealing. I don't really like Teflon tape, it doesn't work as well.

 

edit: ^ beat me to it!

These threads are tapered, so the diameter of the male becomes bigger and the female smaller. The two fittings will tighten up before they bottom out. You should also use thread sealer (teflon tape or such).

Is this what you meant? Also, see:

http://en.wikipedia.org/wiki/National_pipe_thread

 

NPT threads are tapered (that's the "T") so they often do not screw all the way down into the fitting. The exact fit will depend on how they were machined. Always use sealant.

This may help a bit:
http://www.mechanicsupport.com/articleFittingTight.html

I have seen torque spec charts for NPT fittings before but they always end up saying just tighten it until it doesn't leak, then snug it up a bit more.

 

Appreciate the quick answers guys. I thought one of them was the wrong threading