Guides / Process
Thread Standards on Brass Fittings and Which One Seals
September 21, 2026
Almost every machined brass part is a fitting, and almost every fitting fails or succeeds at the thread. Getting the callout right on the drawing prevents most of the arguments that happen later.
The four families you will meet
NPT, National Pipe Taper. American. Tapered, 60 degree thread angle. The taper wedges as you tighten, and the thread itself makes the seal. Usually assembled with PTFE tape or a liquid sealant, because the crests and roots leave a small spiral leak path that the sealant fills.
BSPT, British Standard Pipe Taper, also Rc. Tapered like NPT but with a 55 degree thread angle and a different pitch at most sizes. Seals the same way. It is not NPT.
BSPP, British Standard Pipe Parallel, also G. Parallel. The thread holds the parts together and makes no seal at all. Sealing is done on a face with a bonded seal, an O-ring in a groove or a soft washer.
Metric, M series. Parallel, 60 degree angle. Same principle as BSPP: the thread holds, a face or an O-ring seals. Common on European and Asian equipment and on anything that started as a machine thread rather than a pipe thread.
Why mixing them fails in a way that fools people
This is the expensive one, so it is worth being precise.
An NPT male started into a BSPT female will thread in. It will feel like it is tightening. It will pass a hand check.
It is engaging on partial flanks because the thread angles differ by 5 degrees and, at most sizes, the pitches differ too. The contact area is a fraction of what it should be. Under pressure, or under a thermal cycle, or after a few months, it leaks.
Worse, the failure is intermittent enough that the fitting often gets blamed rather than the mismatch.
The rule is simple. Tapered threads seal against their own standard only. Never mix families, whatever a cross reference chart implies.
Choosing between them for a new design
| Tapered, NPT or BSPT | Parallel, BSPP or metric | |
|---|---|---|
| What seals | The thread plus sealant | A face seal, O-ring or washer |
| Reassembly | Degrades, tape must be redone | Good, replace the seal |
| Orientation control | Poor, it stops where it stops | Good, tighten against the seal |
| Part cost | Lower, no groove or washer | Slightly higher |
| Leak risk from assembly error | Higher, depends on the fitter | Lower |
The practical guidance most designers land on: use tapered where you are matching existing plumbing or where the market expects it, and parallel with a face seal where the joint will be opened and closed repeatedly or where the fitting has to end up pointing in a particular direction.
What this costs you in machining
Threads are not free, and a drawing that says nothing about them gets quoted as though they were.
Each thread is a separate tool and pass, so cycle time goes up. Each sealing thread needs a gauge check, which is inspection time. Tapered threads are checked with go and no-go gauges that measure standoff, and a part that is out is scrap, not rework, because you cannot add material back.
And the thread is usually where the cross hole burr lives. A hole breaking into a threaded bore leaves a burr on the inside that nobody can see and few tools can reach, and that burr is the single most common reason a brass fitting passes at the shop and fails a leak test after assembly. See deburring brass and the cross hole problem.
What to put on the drawing
A thread callout that gets quoted correctly looks like this.
1/2-14 NPT, sealing thread.
Gauge per ASME B1.20.1, go and no-go ring gauge,
standoff within 1 turn.
Bore to be free of burrs at cross hole intersection.
Four things in there, and each one is doing work.
The full designation, not just “1/2 inch thread”.
Whether it seals. A thread that only holds two parts together can be made and checked far more cheaply than one that holds pressure. Tell the shop which it is.
The gauge standard. This turns an opinion into a pass or fail and gets the check built into the process rather than discovered at the end.
The internal burr condition, stated explicitly.
For a parallel thread, add the seal arrangement, because it changes the part:
G1/4 parallel, non sealing.
Seal on face by bonded washer, customer supplied.
Face to be flat within 0.05 mm, Ra 1.6 or better.
One material note
If the part is a yellow brass and the thread is rolled rather than cut, you have introduced cold work and therefore residual stress. On a part that will sit near ammonia or amines, that raises the stress corrosion cracking risk. Either specify cut threads or specify a stress relief afterwards. See season cracking in brass parts.
Have a threaded brass fitting to quote? Send the drawing with the thread callouts in full. Half the RFQs we see are missing the class or the gauge standard, and both slow the quote down.
Frequently asked questions
Can I screw an NPT fitting into a BSPT port?
It will start, and it will not seal reliably. NPT has a 60 degree thread angle and BSPT has 55 degrees, and the pitches differ at most sizes. The threads engage on a few flanks only, which is enough to feel tight and not enough to hold pressure. Treat them as incompatible even though a catalogue sometimes implies otherwise.
Why does BSPP need a washer when NPT does not?
BSPP is a parallel thread. The flanks stay the same distance apart all the way along, so tightening it never creates an interference. The seal is made on a face, by a bonded seal, an O-ring or a soft washer. NPT is tapered, so the flanks wedge together as you tighten and the thread itself forms the seal, helped by a sealant.
Does a sealing thread need a different tolerance class?
It needs the right gauge, which matters more than a tighter number. Tapered pipe threads are checked with go and no-go ring or plug gauges that verify standoff, not with a micrometer. Specify the gauge standard on the drawing and a shop will build the check into the process rather than discovering it at final inspection.
Should threads be cut, rolled or formed on a brass part?
On machined brass fittings threads are normally single point turned or cut with a die head, because the part is already on the lathe. Rolling is used where the thread needs extra strength, but it is cold work, which leaves residual stress and raises the stress corrosion risk on a yellow brass. Say which you want if it matters.
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