What Are Straight Pipe Threads?
Most buyers know NPT, the tapered American pipe thread that seals by wedging the threads together with tape or sealant. ASME B1.20.1 also defines a family of straight pipe threads. They use the same nominal pipe sizes, the same threads per inch and the same 60° thread form as NPT, but the diameter is constant along the length of the thread.
Because there is no taper, a straight pipe thread cannot seal by thread interference. The seal, if one is needed, comes from something else: a gasket, an O-ring, a flared or cone seat, a washer in a hose coupling, or a locknut pressed against a surface. The thread only holds the parts together.
This single difference drives everything else: where the threads are used, how they are toleranced and how they are gauged. For a side-by-side comparison of the main pipe thread families, see our guide to NPT, NPTF, BSPP and BSPT thread gauges.
The Four Straight Pipe Thread Types
The letters follow a simple pattern: N = American National, P = pipe, S = straight, and the last letter describes the application.
| Designation | Full name | Typical use |
|---|---|---|
| NPSM | Straight pipe thread for free-fitting mechanical joints | Fixtures, lamp and valve parts, fittings with a gasket or cone seat |
| NPSL | Straight pipe thread for loose-fitting mechanical joints with locknuts | Joints secured by a locknut; loosest fit of the group |
| NPSH | Straight pipe thread for hose couplings | Hose couplings and nipples sealed by a washer |
| NPSC | Straight pipe thread in pipe couplings | Straight internal threads in couplings mated with NPT external threads |
Typical callouts look like 1/2-14 NPSM, 3/4-14 NPSH or 1-11.5 NPSL. The nominal size and threads per inch match NPT, which is why the two are often confused on the shop floor. A 1/2-14 NPT gauge and a 1/2-14 NPSM gauge look almost identical at first glance, but one is tapered and the other is not.
Do not confuse NPSH with garden hose thread. Garden hose and fire hose threads have their own standards and different pitches. If the part is a hose fitting, check the exact designation on the drawing before you order gauges.
How Straight Pipe Threads Differ from NPT
On an NPT thread, the gauge always screws in; the question is how far. That is why NPT uses L1 gauges and a stand-off tolerance. On a straight thread, the size is the same at every point along the length, so the familiar GO/NO-GO logic applies: the GO gauge must assemble, and the NO-GO gauge must not.
The mating rules also differ. An external NPT thread can often be screwed into an NPSC coupling, and this combination is common in plumbing. An NPSM external thread, on the other hand, is designed to mate with an NPSM internal thread and relies on a separate seal. Mixing families without checking the drawing is a common cause of leaks and of disputes about which gauge is correct.
The NPT side of the story is covered in detail in our article on pipe thread gauge types; the rest of this guide focuses on the straight threads.
Gauging Straight Pipe Threads with GO/NO-GO Gauges
For NPSM, NPSL and NPSH, inspection uses the same principle as unified screw threads under ASME B1.2 practice: a full-form GO gauge checks the functional size, and a short, truncated NO-GO gauge checks the pitch diameter at the least-material limit.
- Internal threads (fittings, couplings, housings) are checked with a GO/NO-GO straight pipe thread thread plug gauge.
- External threads (nipples, hose shanks, threaded bodies) are checked with a GO/NO-GO straight pipe thread ring gauge.
- The GO member must enter or assemble by hand over the length of engagement.
- The NO-GO member may start but must not go beyond the engagement allowed by the applicable gauging practice or your drawing.
| Part feature | Gauge | Good part result |
|---|---|---|
| Internal NPSM thread | NPSM GO/NO-GO plug | GO enters fully; NO-GO does not pass the limit |
| External NPSM thread | NPSM GO/NO-GO ring | GO runs on fully; NO-GO stops early |
| External NPSH hose thread | NPSH GO/NO-GO ring | GO runs on fully; NO-GO stops early |
| Internal NPSL thread | NPSL GO/NO-GO plug | GO enters fully; NO-GO does not pass the limit |
NPSC: The Special Case
NPSC is a straight internal thread intended to mate with an external NPT thread. Because its partner is tapered, NPSC is not always inspected in the same way as NPSM. ASME B1.20.1 describes the gauging method for NPSC, and in practice many drawings and customer specifications refer to taper-type gauging rather than a simple straight GO/NO-GO check.
- Whether NPSC is gauged with a taper plug or straight GO/NO-GO plug
- The acceptance limits your customer applies
- Whether the coupling is made to ASME B1.20.1 or a customer standard
- The mating external thread (NPT or other)
If the drawing does not state how NPSC is to be gauged, we do not guess. Send the drawing or the customer’s inspection instruction, and we will confirm the gauge type with you before quoting.
The same caution applies if you see older or regional variants in a drawing. When the callout is unfamiliar, the safest step is always to ask before any gauge is made.
Where You Will Meet Straight Pipe Threads
- Lighting and electrical fittings — threaded nipples and lamp parts often use straight pipe threads with a locknut.
- Valve and fixture bodies — NPSM threads with a cone or gasket seat.
- Hose couplings — NPSH threads where a washer makes the seal.
- Tank and panel fittings — NPSL threads secured by locknuts through a wall.
- Plumbing couplings — NPSC internal threads receiving NPT pipe.
In most of these products, the thread is a secondary feature from the sealing point of view but a primary feature for assembly. A thread that is slightly oversize may still hold a locknut; a thread that is out of tolerance can let a hose coupling leak because the washer is no longer compressed evenly. Gauging keeps both assembly and sealing predictable.
Common Mistakes with Straight Pipe Thread Gauges
| Mistake | Consequence | How to avoid |
|---|---|---|
| Using an NPT L1 gauge on an NPSM thread | Meaningless stand-off reading | Use straight NPSM GO/NO-GO gauges |
| Using an NPSM gauge on an NPT thread | Tapered part judged as straight | Use L1 taper gauges for NPT |
| Assuming NPSC is gauged like NPSM | Wrong method, disputes with customer | Confirm NPSC method on the drawing |
| Ordering “1/2 pipe thread” with no suffix | Wrong gauge family supplied | Always state NPT, NPSM, NPSH, NPSL or NPSC |
| Confusing NPSH with garden hose thread | Wrong pitch and form | Check the exact hose thread standard |
The simplest safeguard is to mark the gauge case with the full designation, not just the nominal size. Operators who see “1/2 NPSM” on the case are much less likely to pick up the NPT gauge sitting next to it.
What to Send When Ordering Straight Pipe Thread Gauges
- Full callout, e.g.
1/2-14 NPSMor3/4-14 NPSH - Internal, external or both; plug, ring or both
- Quantity of each gauge
- The inspection method your customer specifies, especially for NPSC
- Whether the parts are plated or coated before inspection
- Certificate requirement (ISO/IEC 17025 calibration certificate available on request)
Common sizes may be available from stock; others are made to order. If you have a drawing, attach it — see how to order custom thread gauges for a checklist of what helps us quote accurately.
Need NPSM, NPSH, NPSL or NPSC Gauges?
Send the full thread callout, plug or ring, quantity and your customer’s inspection method. We confirm the gauge type and reply with price and lead time within 24 hours.
Related: NPT vs NPTF vs BSP gauges · Thread ring gauges · Thread plug gauges
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GageTools supplies GO/NO-GO plug and ring gauges for NPSM, NPSH and NPSL straight pipe threads, plus NPT, NPTF and BSP pipe thread gauges. ISO/IEC 17025 calibration certificate available on request. Standard sizes ship from stock within 72 hours; custom gauges in 10–15 working days. MOQ 1 piece. WhatsApp for fastest response.
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