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Can I screw a hose fitting onto a pipe thread?
A hose thread and a pipe thread can share a nominal size yet seal in opposite ways, so a hose fitting does not belong on a pipe thread.
Straight versus tapered
NPSH is a straight hose thread. Its diameter stays constant along the thread and it seals on a washer at the face, the same way a garden hose does. NPT is a tapered pipe thread. Its diameter narrows along the thread so the flanks wedge together and seal on the thread itself.
Because one seals on a washer and the other on the thread, they are built for different jobs even when the nominal size matches.
Why they seem to fit
An NPSH and an NPT part of the same nominal size will often start threading together, which is exactly what tempts people to force them. A straight hose fitting pushed onto a tapered pipe thread will bind partway, sit crooked, and either leak or crack the fitting.
Reverse it and a tapered pipe fitting into a straight hose seat will not pull up against a washer properly, so it weeps.
Identifying which is which
Look for taper. Run a straightedge along the crests: NPT visibly narrows toward the end, NPSH stays parallel. Look for a washer seat too, because a hose thread usually has a flat face and a washer while a pipe thread has neither and expects sealant on the thread.
If a part came off a hose, garden tap or appliance inlet, suspect a hose thread. If it came off pipework or a pump body, suspect NPT.
Bridging correctly
To join a hose to pipework, use an NPSH-to-NPT adapter, sealing the NPT side on the thread with PTFE tape and the NPSH side on a washer at the face. That way each end seals the way it was designed to, and you avoid the cross-thread damage that comes from forcing a hose fitting onto a pipe thread.
Get the right fitting
Open the HoseFitFinder toolMatch hose thread to pipe threadFind NPSH to NPT hose adapter on eBayaffiliateRelated
Sources: Sheldon Brown plumbing reference