The black piece labeled "NPT" is the water connections to a Clack water softener system. I need to attach it to PEX-A tubing and found this brass part that fits. Is it safe to attach these two parts together? Should I be using Teflon tape or similar?
The OP's top photo shows a black plastic fitting that seems to be straight thread, not tapered threads. Thus, it is likely mislabeled, and not actually "NPT"
According to the usplastic.com website:
NPT (National Pipe Thread) and NPS (National Pipe Straight) have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered, and NPS threads are straight (parallel). Both threads have a 60° included angle and have flat peaks and valleys
It is the tapered threading of NPT fittings that makes them water-tight, as the metal threading is forced together with tightening. Teflon tape and/or pipe dope help lubricate the threads and fill any tiny gaps. Continued tightening of this type of joint improves sealing as long as the metal does not crack under excess pressure.
Straight-thread, especially plastic, will not make a seal in this fashion. Instead, straight-thread fittings seal by compressing a flexible gasket/washer at the round flat face inside the female end of the fitting, where the male fitting's round end threads in. Over-tightening this type of joint will normally cause a leak as the flexible/rubber gasket is distorted by the overpressure. This type of joint should normally be tightened by hand until hand-tight, then further tightened perhaps 2/3 of a turn to get the right compression.
You need to find a female straight-thread adapter that matches the threading on your plastic fitting and that has a rubber washer inside for a proper end-seal. A plumbing supply house or hardware store should be able to help.
Usually, a threaded coupling the way you propose is the only way to connect two different types of pipe. Sometimes there is a specific fitting made for specific applications (e.g. some pre-made push-on to plastic or pex), but they all serve the same purpose. A common third party that supports connection to both sides.
Make absolutely sure the threads are the same type on both sides and you will be good to go. Male NPT goes to Female NPT, Male NPS goes to Female NPS, and the same goes for hose thread, flare, etc. Do not mix thread types even though they look similar -- a 3/4" MNPT might fit inside a 3/4" FHT but they will never seal properly.
I do use a wrap or two of Teflon tape as well - it helps fill the little gaps that can arise when you have threads of two different material hardness mating.
