Guide
What is the difference between NPT and NPTF Dryseal pipe threads?
NPTF is the Dryseal American Standard taper pipe thread defined in ASME B1.20.3: a thread with the same taper, pitch and basic diameters as ordinary NPT, but with the crest and root flats held to tighter, coordinated limits so that the joint seals metal to metal, without tape or sealing compound.
By Saola Manufacturing engineering team · Published · Updated
NPT, covered by ASME B1.20.1, is the general-purpose American taper pipe thread. Its crests and roots may vary over a wide band, leaving a small helical gap that a sealant has to fill; NPTF (National Pipe Taper Fuel, the Dryseal thread), covered by ASME B1.20.3, closes that gap by design. The two look identical, which is why they get mixed up on drawings and at incoming inspection. The values below come from the public federal edition of the pipe-thread standards; cite the current ASME documents on a drawing.
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How does an NPTF thread seal without tape or sealant?
An NPTF thread seals because its truncation limits are coordinated so that the crest of one member meets the root of the other at the same moment as, or before, the flanks touch. With crests, roots and flanks all in contact after wrench makeup, there is no spiral path left for fluid to follow.
Every taper pipe thread seals on an interference fit that grows as the joint is tightened, because both members are cut on the same 1 in 16 taper per ASME B1.20.1. On standard NPT the flanks are meant to carry that contact and the crests and roots are left loose. The federal handbook that mirrors the ASME text says it plainly: an NPT joint is made up wrench tight with lubricant or sealer, and the flanks are the surfaces intended to meet (NBS Handbook H28, Part II). The sealer fills the triangle of space left at the crest and root.
Dryseal removes that triangle. Per ASME B1.20.3, the crest of each thread is truncated less than the mating root, and the two limits meet: in every pitch in the table below, the largest permitted crest flat equals the smallest permitted root flat. When the joint is tightened, the sharper crest is forced into the wider root and deforms slightly, sealing the helix.
| Threads per inch | NPT crest or root truncation (in) | NPTF crest truncation (in) | NPTF root truncation (in) |
|---|---|---|---|
| 27 | 0.0012 to 0.0036 | 0.0017 to 0.0035 | 0.0035 to 0.0052 |
| 18 | 0.0018 to 0.0049 | 0.0026 to 0.0043 | 0.0043 to 0.0061 |
| 14 | 0.0024 to 0.0056 | 0.0026 to 0.0043 | 0.0043 to 0.0061 |
| 11.5 | 0.0029 to 0.0063 | 0.0035 to 0.0052 | 0.0052 to 0.0078 |
| 8 | 0.0041 to 0.0078 | 0.0052 to 0.0069 | 0.0069 to 0.0095 |
Read across a row: an NPT crest can be cut near the top of its band while the mating root sits near the bottom, and the difference is the leak path. An NPTF crest can never be flatter than the mating root.
Are NPT and NPTF threads interchangeable?
Mechanically, yes: NPT and NPTF share the thread angle, taper, pitch and basic pitch diameters, so an NPT fitting will screw into an NPTF port and the reverse. The dry seal does not survive the swap, so a mixed joint has to be treated as an NPT joint and sealed with compound or tape.
The Dryseal feature depends on both members having Dryseal truncation. If one side is cut to the NPT band, nothing guarantees that its crest reaches the other side’s root. The federal handbook makes the same point from the other direction: trouble-free joints without lubricant or sealer are best assured when both components carry full-length NPTF threads (NBS Handbook H28, Table VIII.3 notes).
NPTF is also not the only Dryseal thread. ASME B1.20.3 defines four families, and the symbol on the drawing says which one is meant:
- NPTF — full-length tapered external and internal threads, the general Dryseal thread and the one to choose when in doubt.
- PTF-SAE SHORT — tapered, with one thread removed from the full length (at the small end of the external thread, the large end of the internal one) for clearance and less material.
- NPSF — a straight internal fuel thread, generally used in soft or ductile materials that adjust to the taper of the external thread at assembly.
- NPSI — a straight intermediate internal thread, generally used in hard or brittle materials with heavy sections that expand little at assembly.
The handbook’s interchangeability table shows which pairs are intended to assemble; Dryseal threads are never used where both members are straight.
How are NPTF threads gauged, and why does that cost more than NPT?
NPTF is gauged with two thread gauges per member instead of one, with gauge crests truncated to clear the Dryseal flats, and the crest and root truncation must also be checked separately, usually by optical projection. That extra hardware, calibration and inspection time is where most of the cost difference between NPT and NPTF comes from.
A standard NPT thread is typically accepted with a single L1 plug or ring: the gauge is screwed on hand tight, and the position of its notch or face against the part must fall within one turn of basic. NPTF keeps that hand-engagement check and adds a second gauge to control taper and diameter over the rest of the thread. Per the federal Dryseal gauging section, an external thread is checked with a thin L1 ring and a full-length L2 ring, and an internal thread with an L1 plug and an L3 plug that checks the wrench-makeup length (NBS Handbook H28, Section VIII.8).
| Inspection item | NPT | NPTF (Dryseal) |
|---|---|---|
| External thread gauges | L1 ring | L1 thin ring plus L2 full ring |
| Internal thread gauges | L1 plug | L1 plug plus L3 plug |
| Gauge thread crests | Standard pipe-thread form | Truncated 0.20p to 0.25p to clear the Dryseal flats |
| Product tolerance at gauge notch | 1 turn large or small | 1 turn large or small |
| Crest and root truncation | Guide limits for tooling | Controlled; checked by projection or truncation gauges |
| Sealant at assembly | Expected | Not required; lubricant allowed against galling |
The thread gauges alone do not prove the Dryseal feature, because they sit on the flanks and do not see the flats. The handbook strongly recommends projecting the threads of the tools, and of a share of the product, to confirm crest and root truncation, with special plug and ring gauges as the alternative. On a machined port, that usually means controlling truncation through the tap or insert form and checking the tool, not every part.
Gauge standoff is the other trap. Two gauges that are each within their own tolerance can still disagree with each other; the handbook tabulates the maximum possible standoff between any plug and any ring at opposite tolerance limits, and recommends measuring each working gauge against its reference gauge and retiring it once it wears past half a turn from basic. Two inspection rooms using different working gauges can reach different verdicts on the same part, so ask the supplier which reference gauges its working gauges are checked against.
When should a drawing call out NPTF instead of NPT?
Call out NPTF when the joint must be pressure tight and sealant is unwelcome: fuel, hydraulic, gas and refrigerant lines, and any system where sealant particles or tape shreds could reach a valve seat, orifice or sensor. If sealant is acceptable and the joint is reassembled rarely, NPT with a compound is usually the simpler and cheaper choice.
The federal handbook lists refrigerant, marine, automotive and aircraft fuel and oil line fittings among the thread’s principal early uses: fluids that are valuable, flammable or sensitive to contamination.
There are also good reasons not to specify it. NPTF narrows the tolerance on every threaded member and doubles the gauge set, so it should not appear on drawings by habit. Where the design already has a second sealing element, such as an O-ring boss port or a face seal, the tapered thread is only holding the part together, and NPT or a straight thread is the better choice. Where the joint is made up and broken often, ask the fitting maker how many reassemblies the seal is meant to survive, because the seal relies on crest deformation at makeup.
If you do specify it, write the full designation that the standard uses, for example “1/4-18 DRYSEAL NPTF”, and say whether sealant is permitted at assembly.
What changes when the NPTF port is in a die casting?
The thread can seal perfectly and the part can still leak, because machining a port into a die casting can open internal porosity that runs past the threads. A Dryseal callout on a cast housing therefore needs a leak or pressure test, and often impregnation, alongside the thread gauge check.
High-pressure die castings have a dense skin and a core that may hold gas or shrinkage porosity. Tapping a pipe port cuts through the skin, and a pore that connects the thread to the outside of the wall, or to a second passage, becomes a leak path that no thread form can close. The usual remedy is vacuum impregnation, in which a sealant is drawn into the pores of the finished casting. The military design criteria for impregnating porous nonferrous castings, including aluminum and zinc, are set out in MIL-STD-276A, and NADCA’s design guidance notes that cost is kept down by pressure testing every casting and impregnating only those that fail (NADCA, secondary processing).
Material matters to the thread itself as well. For straight Dryseal ports, the federal handbook notes that taps that produce holes to size in cast iron, steel and brass produce holes about 0.001 in. smaller in zinc and similar soft metals, and that plug-gauge turns engagement should be reduced to suit (NBS Handbook H28, Tables VIII.7 and VIII.8 notes). That matters for straight ports in Zamak housings, where a tap chosen for brass will leave the port tight. For hard or brittle materials with thin sections, the same handbook says the tapered internal NPTF thread minimizes trouble from fracture.
For a cast part, three items belong on the drawing with the thread callout: which alloy the port is in, the leak or pressure test and its acceptance limit, and whether impregnation is allowed, required or prohibited. The port itself is normally cut after casting on a CNC milling operation, so place it clear of thick sections and gate areas where the die casting is most likely to hold porosity.
What should an RFQ for NPTF parts sent to an overseas supplier include?
Send the full Dryseal designation, the gauging method and standard edition, the sealant rule at assembly, any coating, and the leak test, so the supplier can price gauges and inspection instead of discovering them after the order. For a supplier in Vietnam or anywhere else outside North America, also ask which Dryseal gauges it already holds.
- Designation. “DRYSEAL NPTF” with size and threads per inch, citing ASME B1.20.3; add PTF-SAE SHORT, NPSF or NPSI only if you mean them.
- Gauging. Name the method, such as position gauging with basic-notch gauges, and say whether truncation is verified by projection of the tool or of sample parts.
- Gauge ownership and traceability. Inch Dryseal gauges are less common in shops that mostly cut metric and BSP threads, so ask whether the supplier owns L1, L2 and L3 gauges in each size and how they are calibrated. If you supply master gauges, say so in the RFQ.
- Coatings. State whether the thread is gauged before or after plating or anodizing, or masked.
- Leak test. Test medium, pressure and acceptance limit, especially on cast parts.
Turned fittings with Dryseal threads are ordinary CNC turning work once those points are settled; the thread itself is held to the standard’s limits, and the tolerances on the rest of the part are confirmed in the quote. The RFQ package guide covers the rest of what to send. When the drawing is ready, send it for a quote and the reply will list any thread, gauging or test question that needs an answer first.
Frequently asked
What does “1/8-27 DRYSEAL NPTF” mean?
Nominal pipe size 1/8, 27 threads per inch, Dryseal form, NPTF series. In the symbol, N stands for American (National) Standard, P for pipe, T for taper and F for fuel and oil, the service the thread was designed for.
Can I still use PTFE tape or sealant on an NPTF joint?
The standard does not require it, and allows a lubricant where it does no harm, to prevent galling. Some assemblers add a sealant as a backup anyway. Either way, write the rule on the drawing or assembly instruction so the decision is not made on the line.
If a part passes an NPT gauge, is it good NPTF?
Not necessarily. An NPT L1 gauge checks where the thread sits on its taper, not how its crests and roots are truncated, and truncation is the Dryseal feature. NPTF needs its own gauge set plus a truncation check.
Is NPTF the same as BSPT or an ISO 7 thread?
No. BSPT and ISO 7-1 threads use a 55-degree thread angle and different pitches in several sizes, while NPT and NPTF use 60 degrees. They do not interchange and they seal differently, so the thread standard must be named on the drawing.
Does zinc plating or anodizing affect a Dryseal thread?
Yes. Any coating on the flanks moves the gauge position and can fill the crest and root clearances the seal depends on. Decide whether the thread is gauged before or after finishing, or masked, and put that on the drawing.
Sources
- ASME B1.20.3-1976 (R2023), Dryseal Pipe Threads, Inch, ASME, 1976, reaffirmed 2023
- ASME B1.20.1-2013 (R2018), Pipe Threads, General Purpose, Inch, ASME, 2013, reaffirmed 2018
- Handbook H28, Screw-Thread Standards for Federal Services, Part II: Pipe Threads, including Dryseal Pipe Threads (Sections VII and VIII), National Bureau of Standards (now NIST), 1957
- MIL-STD-276A, Design Criteria: Impregnation of Porous Nonferrous Metal Castings, Department of Defense, 30 December 1992 (public copy)
- Secondary Processing, NADCA Design, North American Die Casting Association, accessed September 2026