Guide
NPS vs NPT: when should a machined port use an NPSM or other NPS thread instead of tapered NPT?
NPS thread is the family of American National Standard straight (parallel) pipe threads, designated NPSC, NPSM, NPSL, NPSH and the Dryseal NPSF and NPSI, that keep the thread angle and pitch of tapered NPT but have no taper, so most of them hold parts together while a gasket, O-ring, seat or sealant does the sealing. NPT seals on the thread itself, because its taper wedges the flanks tight as the joint is made up.
By Saola Manufacturing engineering team · Published · Updated
The short rule for a drawing: use NPT (or Dryseal NPTF) when the thread must be the pressure seal, use NPSM or NPSL only when something else seals or nothing needs sealing, and use a straight-thread O-ring port instead of either when a hydraulic manifold has to stay leak-free and be reassembled. The sections below explain each suffix, which pairs actually screw together, how the sizes relate, and how each one is called out, tapped and gauged. The tabulated values come from the 1957 federal handbook that printed the predecessor ASA pipe-thread standards, checked against the current ASME scopes; cite the current ASME documents on the drawing.
On this page
What is the difference between NPS and NPT threads?
NPT is a tapered pipe thread that seals by wedging its flanks together with sealant in the gaps, while NPS threads are straight versions of the same thread form that seal only with help from a gasket, O-ring, cone seat, locknut washer or, in the NPSC case, a tapered NPT mating part and sealant. The letters tell you which: N is American National, P is pipe, T is taper and S is straight.
Every American pipe thread uses the same 60-degree thread form and the same threads per inch for a given nominal size, which is why the two families look alike and get swapped by mistake. The difference is the diameter along the length. An NPT thread grows on a taper of 1 in 16 on diameter, per ASME B1.20.1, so tightening forces an interference fit. A straight NPS thread has a constant pitch diameter, so it spins freely until a shoulder, seat or nut stops it, and it cannot close its own helical leak path.
The 1957 federal handbook, which printed the ASA standards that later became the ASME documents, describes the straight types by purpose (NBS Handbook H28, Part II, Sections VII, VIII and X):
- NPSC — straight internal thread in pipe couplings. It forms a pressure-tight joint only with a tapered NPT external thread and lubricant or sealer, and the handbook recommends the joint for comparatively low pressures only.
- NPSM — free-fitting straight thread for mechanical joints, external and internal, written for assemblies made in the shop where a reasonably close fit is wanted and, in the handbook’s words, where there are no internal pressures.
- NPSL — loose-fitting straight thread for locknut connections, the largest diameter that can be cut on standard pipe; the handbook’s example is the nipple joint that connects a standpipe through the floor or wall of a water tank.
- NPSH — straight thread for hose couplings that join taper pipe threads with a gasket, now covered in ASME B1.20.7, the hose coupling screw thread standard.
- NPSF and NPSI — Dryseal straight internal threads from ASME B1.20.3, made to seal metal to metal against a tapered NPTF external thread.
Which NPS and NPT threads can be assembled together?
A tapered external thread can go into a straight internal thread, and that is how NPSC, NPSF and NPSI are meant to be used, but a straight external thread should not be put into a tapered port. Two straight threads only ever make a mechanical joint, so the seal has to come from a gasket, O-ring or seat.
The table lists the pairings the standards intend and the ones that only look as if they work. The word “assembles” here means the threads engage; whether the joint holds pressure is the last column.
| External (male) thread | Internal (female) thread | Intended pairing? | What makes the seal |
|---|---|---|---|
| NPT | NPT | Yes, the general case | Thread interference plus tape or compound |
| NPT | NPSC | Yes, low pressure only | Thread plus lubricant or sealer |
| NPTF | NPTF, NPSF or NPSI | Yes, Dryseal | Crest and root metal contact; sealant not required |
| NPTF | NPT | Assembles | Treat as NPT: sealant required |
| NPSM | NPSM | Yes, mechanical joint | Gasket, O-ring or seat; none from the thread |
| NPT or NPTF with cone seat | NPSM swivel nut | Yes, common hose end | Cone seat inside the male fitting |
| NPSL | NPSL locknut | Yes, loose fit | Gasket under the locknut |
| NPSM | NPT | No | None; binds within about a turn |
| NPSL | NPT | No | None; pitch diameter is above E1, so it may not start |
| NPT | NPSM | Not as a seal | None from the thread; jams on the taper |
The common field case is the hydraulic hose end. A female NPSM swivel nut is screwed onto a male NPT or NPTF fitting whose bore carries a 30-degree cone seat, and the seal is made where the swivel’s nose meets that cone; the straight thread only clamps the two together. A plain NPT male without the seat will accept the nut but will not seal, so a machined adapter meant for NPSM swivels needs the cone on its drawing. Fitting makers differ on whether sealant is also applied, so follow the mating fitting’s instructions.
How do NPSM and NPSC sizes relate to NPT?
Both are built around the pitch diameter of NPT at its hand-tight gauging plane, called E1: the NPSM internal thread starts at E1 and goes larger, the NPSM external thread sits just below it, and the NPSC internal thread straddles it. That is why an NPT male enters an NPSM or NPSC port by roughly its hand-tight length and then wedges.
Per the federal handbook, the basic pitch diameter of all the straight mechanical-joint threads equals the NPT pitch diameter at the gauging notch, which is also the large end of an internal taper thread (NBS Handbook H28, Section VII.6). The NPSM external thread is given an allowance below that so the parts fit freely, and the NPSC tolerance band corresponds to one and one-half turns large or small of a standard taper thread.
| Size and threads per inch | NPT E1, basic (in) | NPSC internal (in) | NPSM internal (in) | NPSM external (in) |
|---|---|---|---|---|
| 1/8-27 | 0.3736 | 0.3701 to 0.3771 | 0.3736 to 0.3783 | 0.3689 to 0.3725 |
| 1/4-18 | 0.4916 | 0.4864 to 0.4968 | 0.4916 to 0.4974 | 0.4859 to 0.4903 |
| 3/8-18 | 0.6270 | 0.6218 to 0.6322 | 0.6270 to 0.6329 | 0.6211 to 0.6256 |
| 1/2-14 | 0.7784 | 0.7717 to 0.7851 | 0.7784 to 0.7851 | 0.7718 to 0.7769 |
| 3/4-14 | 0.9889 | 0.9822 to 0.9956 | 0.9889 to 0.9958 | 0.9820 to 0.9873 |
| 1-11.5 | 1.2386 | 1.2305 to 1.2468 | 1.2386 to 1.2462 | 1.2311 to 1.2369 |
Read the last two columns against the first. An NPSM male is always smaller than E1, and an NPT port is at E1 only at its mouth and gets smaller inward, so a straight male screwed into a tapered port binds after very little engagement and leaves the joint loose and unsealed. Going the other way, an NPT male goes into an NPSM port about as far as it would go hand tight into an NPT port, then jams on the taper; that is acceptable for an NPSM swivel that seals on a cone, but it is not a pressure joint by itself.
How are NPS and NPT threads called out, tapped and gauged?
Write the size, threads per inch and series symbol, such as 1/2-14 NPT or 1/2-14 NPSM, and add the class and gauging method for straight threads. NPT ports are cut with taper pipe taps and gauged with a taper plug, while NPSM ports are cut with straight pipe taps and gauged with straight go and not-go plugs.
The designation format in the standards is size, pitch and symbol in sequence, for example “3/8-18 NPSM”, “1/8-27 NPSC” or “1/4-18 DRYSEAL NPSF”. Left-hand threads add “-LH”. In the federal tables, NPSM product limits are listed as class 2A external and class 2B internal; the designation itself carries no class, so a note naming the class removes one more question.
Gauging follows the shape of the mating part. The handbook calls for straight go and not-go gauges wherever both threads are straight, and allows the regular taper plug for straight internal threads whose mating external thread is tapered; for NPSC that taper plug must sit flush with the end of the coupling, or the bottom of the chamfer, within one and one-half turns large or small (NBS Handbook H28, Section VII.8). An NPT port is checked with the L1 taper plug at plus or minus one turn from the notch, as the NPT and NPTF guide explains.
On the machine, the practical differences are these:
- Tool. NPT and NPTF need a tapered tap or a thread mill programmed on the taper; NPS ports use a straight pipe tap or a straight thread-milling path, the same as any parallel thread.
- Depth. On a tapered port, depth controls size: a tap run too deep makes the port oversize at the gauge. On a straight port, depth only sets length of engagement, and a shoulder or seat face usually sets the stop.
- Seal face. A straight port almost always needs a second feature on the drawing, such as a spot face, gasket land, cone seat or O-ring chamfer, with its own finish and runout.
- Gauges. A buyer sourcing NPSM or NPSC ports should confirm the supplier holds the right gauge type; an NPT plug does not prove an NPSM port, and the handbook specifies the taper plug, not straight gauges, for NPSC.
Should a machined manifold port be NPT, NPSM or an SAE J1926 O-ring boss?
For a hydraulic manifold, choose a straight-thread O-ring port: metric ISO 6149 for new hydraulic designs, as the SAE J1926-1 scope itself directs, or the inch SAE J1926-1 port where the system already uses inch O-ring boss fittings. Keep NPT for low-pressure air, water, gas and instrument ports where sealant is acceptable, and use NPSM only where a swivel, gasket or face seal carries the pressure.
An O-ring boss port seals with an O-ring compressed in a chamfer at the mouth of a straight UN thread, so the thread carries load and the elastomer carries pressure, and the fitting can be oriented and reassembled without re-doping threads. The SAE scope states that these ports may be used at working pressures up to 63 MPa for nonadjustable stud ends and 40 MPa for adjustable ones, and that new hydraulic fluid power designs should use ISO 6149 ports (SAE J1926/1, 2023 revision). A tapered NPT port in the same block relies on sealant and on wedging the body, so it is less suited to joints that are taken apart and remade.
| Port situation | Specify | Why |
|---|---|---|
| Hydraulic fluid power, pressure tight, reassembled in service | ISO 6149 (metric) for new designs; SAE J1926-1 (inch) to match existing inch fittings | Elastomer seal, orientable fittings, no thread sealant |
| Air, water, gas or instrument port; sealant acceptable | NPT | Simple taper seal, widest fitting availability |
| Fuel, refrigerant or sealant-free port; both members Dryseal | NPTF, or NPSF or NPSI internal with NPTF male | Metal-to-metal thread seal |
| Male connection on a machined part feeding a hose with an NPSM swivel | NPT or NPTF male with the cone seat drawn | Swivel seals on the cone, not the thread |
| Mechanical attachment, no internal pressure | NPSM, class 2A and 2B | Free fit, repeatable assembly |
| Nipple clamped through a wall or panel with a locknut | NPSL with gasket | Loose fit, seal at the gasket |
When we review a manifold drawing, the question we ask first is what seals each port, because the answer decides the thread, the tooling and the gauge. O-ring ports are cut on CNC milling centers, often with a port-forming tool that cuts the hole, O-ring chamfer and spot face together before the thread is tapped or milled, and turned fittings with NPT, NPTF or NPSM threads and cone seats are routine CNC turning work. The O-ring groove guide covers the elastomer side of the joint.
What should an RFQ for parts with pipe-thread ports include?
Give the full thread designation with the governing standard, the sealing method for each port, the gauge type and acceptance rule, any seat or spot-face geometry, and the leak test if one applies. With those stated, the supplier can quote the right taps and gauges instead of guessing between NPT and NPS.
- Designation. Size, threads per inch and symbol, for example 1/2-14 NPSM with a note that the internal thread is class 2B, citing ASME B1.20.1 (or B1.20.3 for Dryseal, SAE J1926-1 for O-ring ports).
- Seal. Say whether sealant is allowed, and draw the seat, gasket land or O-ring chamfer that carries the pressure on straight ports.
- Gauging. Taper plug with turns tolerance for NPT and NPSC, straight go and not-go for NPSM and NPSL, and who supplies master gauges if any.
- Coating. Whether the thread is gauged before or after plating or anodizing, or masked; see the guide on threads after anodizing and plating.
- Test. Medium, pressure and acceptance limit for any leak or proof test.
If a legacy drawing just says “1/2 NPS” with no suffix, ask which series is meant before machining, because NPSC, NPSM and NPSL are different sizes and a wrong guess only shows up at assembly. When the package is ready, send it for a quote and any open thread or gauging question will be listed in the reply.
Frequently asked
What does NPSM stand for?
National Pipe Straight Mechanical: American Standard straight pipe thread for free-fitting mechanical joints. It shares the NPT thread form and pitch but has no taper, so it does not seal on the thread.
Is “NPS” the same as nominal pipe size?
Not on a thread callout. In pipe dimension standards NPS means nominal pipe size, the size name of the pipe; in a thread designation the letters begin a straight pipe thread symbol such as NPSM or NPSC. A callout that reads only “NPS” is incomplete and should be queried.
Will an NPT fitting screw into an NPSM port?
It will start and run in roughly as far as it would hand tight in an NPT port, then wedge on its taper. The joint holds the parts together but does not seal unless a cone seat, gasket or O-ring is designed in.
Can thread tape make an NPSM joint seal?
Not reliably. A straight thread has no interference to compress the tape, so the designed seal on an NPSM joint is the seat, gasket or O-ring; follow the fitting maker’s assembly instructions.
Is NPSF the same as NPSM?
No. NPSF is a Dryseal straight internal fuel thread from ASME B1.20.3, held to crest and root limits so it seals metal to metal against an NPTF male; NPSM is a free-fitting mechanical thread from ASME B1.20.1 with no sealing function.
Sources
- ASME B1.20.1-2013 (R2018), Pipe Threads, General Purpose, Inch, ASME, 2013, reaffirmed 2018
- ASME B1.20.3-1976 (R2023), Dryseal Pipe Threads, Inch, ASME, 1976, reaffirmed 2023
- Handbook H28, Screw-Thread Standards for Federal Services, Part II: Pipe Threads (Sections VII and VIII), National Bureau of Standards (now NIST), 1957
- SAE J1926/1, Ports and Stud Ends with ASME B1.1 Threads and O-Ring Sealing, Part 1: Threaded Port with O-Ring Seal in Truncated Housing, SAE International, revised January 2023
- ASME B1.20.7, Hose Coupling Screw Threads (Inch), ASME, 1991, reaffirmed 2024