Pipe, Dryseal and Special-Purpose Threads: Type 3

Type 3 — NPSF: Straight (Cylindrical) Internal Thread

NPSF threads are straight (not tapered) and are internal only. They are more economical to produce than tapered internal threads. When assembled with a tapered external thread:

• Sealing occurs at only one thread (the thread where interference first occurs as the taper engages the parallel bore)
• The degree of sealing is dependent on material ductility — soft or ductile materials tend to adjust at assembly to the taper of external threads, improving the seal
• Hard or brittle materials are acceptable only where the section is thick enough to allow the necessary thread deformation

NPSF threads are not as reliable as fully tapered NPTF assemblies.

Type 4 — NPSI: Straight Cylindrical Internal Thread (Slightly Larger)

NPSI threads are like NPSF but with slightly larger diameters — though both have the same tolerance and thread length. NPSI is used where there is little expansion at assembly with external taper threads — hard or brittle materials where NPSF's interference might crack the part.

Like NPSF, NPSI does not provide as strong a guarantee of sealing as a tapered internal thread.

Limitation of Assembly Among Dryseal Thread Types

This matrix governs which external thread types can be assembled with which internal thread types — and under what conditions:

At extreme tolerance limits, shortened thread lengths reduce hand engagement and threads may not start assembly. When straight internal threads are used, interference occurs at only one thread.

Designation of Dryseal Pipe Threads

Dryseal threads are designated by nominal size, thread series symbol, and class:

1⁄8-27 NPTF-1
1⁄8-27 PTF-SAE SHORT
3⁄8-18 NPTF-1 AFTER PLATING

Suggested Tap Drill Sizes for Internal Dryseal Pipe Threads

Because Dryseal threads achieve sealing by root and crest interference, the tap drill selection for internal dryseal threads requires special consideration. Both straight (NPSF/NPSI) and tapered options are covered:

Best practice: Ream after drilling whenever possible for taper pipe tap holes. A reamer with a taper of 3⁄4 inch per foot is recommended for best Dryseal sealing performance.

Special Dryseal Thread Series

When standard NPTF or PTF-SAE SHORT threads cannot be used due to design constraints — thin wall sections, permanent installations, tight space envelopes — three additional Dryseal series are available:

PTF-SPL SHORT

Conforms in all respects to PTF-SAE SHORT threads except that the full thread length has been further shortened by eliminating one additional thread:

• For internal threads: one more thread removed at the small end
• For external threads: one more thread removed at the large end

PTF-SPL EXTRA SHORT

Conforms to PTF-SAE SHORT except two additional threads are eliminated:

• For internal threads: two more threads at the small end
• For external threads: two more threads at the large end

F-PTF — Dryseal Fine Taper Thread Series

Developed for applications requiring finer pitches on nominal pipe sizes. The series applies 27 threads per inch to 1⁄4 and 3⁄8 pipe sizes, and recommends applying the existing thread pitch to the next larger pipe size:

1⁄4-27,  3⁄8-27,  1⁄2-18,  3⁄4-18,  1-14,  1¼-14,  1½-14,  2-14

Applies to both external and internal full-length threads. Suitable where threads finer than standard NPTF are required.

SPL-PTF — Dryseal Special Diameter-Pitch Combination Series

Used where taper pipe threads are applied to nominal-size thin-wall tubing. Diameter-pitch combinations in this series:

1⁄2-27,  5⁄8-27,  3⁄4-27,  7⁄8-27,  1-27

The OD of the tubing is specified in the designation. For example:

1⁄2-27 SPL-PTF, OD 0.500

Designation of Special Dryseal Threads

1⁄8-27 PTF-SPL SHORT
1⁄8-27 PTF-SPL EXTRA SHORT
1⁄2-27 SPL-PTF, OD 0.500

Assembly Limitations for Special Dryseal Combinations

The following assembly matrix applies when Special Short or Extra Short Dryseal threads are to be combined:

Interpretation: Special short threads are conditional assemblers. They are not interchangeable with standard full-length NPTF in high-pressure applications without engineering review.

British Standard Pipe Threads

British and European practice uses a parallel (Whitworth-form) thread system for pipe, governed by two distinct British Standards that address fundamentally different sealing requirements.

BS 21:1973 — Pipe Threads for Pressure-Tight Joints (BSP-Taper / BSP-Parallel)

These threads are Whitworth-form and are specified in two joint types:

1. Jointing Threads (Taper for Pressure Sealing):

Taper external threads for assembly with either taper or parallel internal threads. The mating of the threads itself produces the pressure-tight joint. Parallel external threads are not suitable as jointing threads — only taper external threads make the seal.

2. Longscrew Threads (Parallel External, Pressure via Backnut):

Parallel external threads used for longscrews (connectors). A pressure-tight joint is achieved by compression of a soft material onto the surface of the external thread by tightening a back nut against a socket. The thread itself does not seal — the compression of the packing material does.

BS 2779:1973 — Pipe Threads for Non-Pressure-Tight Joints ("G" Thread)

These are Whitworth-form parallel fastening threads used for mechanical assembly of fittings, cocks, and valves where no pressure sealing is required.

Two tolerance classes for external threads:

• Class A: Tighter tolerance, entirely negative (same magnitude as internal thread tolerance). For applications where close tolerance is essential.
• Class B: Tolerance twice that of Class A, also entirely negative. Preferred for economy of manufacture. Should be chosen whenever possible.

One tolerance class for internal threads (no class letter designation required).

Drawing call-out convention:

G1⁄2           →  internal thread (no class needed)
G1⁄2 A         →  external thread, Class A
G1⁄2 B         →  external thread, Class B (assumed if no class stated)
G1⁄2 T         →  internal thread, truncated
G1⁄2 BT        →  external thread, Class B, truncated

The truncation designation T is added when the Whitworth crest is truncated to the limits specified in BS 2779.

Hose Coupling Threads — NH, NHR, NPSH

Hose connections form their own thread family, standardized in ANSI/ASME B1.20.7-1991, because garden hose, fire hose, industrial air hose, and steam hose all impose unique requirements on thread geometry.

The three standard ANSI hose thread series:

NH (SPL) — a special variant for marine applications.

Hose coupling thread dimensions — NH (NFPA Standard):

Tolerance relationships for NH threads:

$$\text{Minor Diameter Tolerance} = 2 \times \text{Pitch Diameter Tolerance}$$

$$\text{Major Diameter Tolerance} = \text{Pitch Diameter Tolerance} - \frac{2h}{9}$$

The minimum minor diameter of the internal thread produces a basic flat of $p/8$ at the crest when pitch diameter is at minimum. The maximum major diameter of the coupling corresponds to a flat of $p/24$ at the major diameter.

The "Higbee Cut" — Fire Hose Thread Protection

American National Fire Hose Connection Thread (NFPA No. 194-1974) governs the connections used on fire apparatus — hose couplings, suction hose, relay supply hose, fire pumps, hydrants, nozzles, and standpipe systems.

This thread employs the blunt start ("Higbee Cut") — an engineered interruption on full thread at both external and internal thread ends. Without this cut, fire hose threads under the chaos of rapid deployment (often in darkness, under pressure, by fatigued personnel) can cross-thread, damaging the coupling and creating a potentially fatal failure during active firefighting.

The Higbee Cut ensures that threads cannot cross — they either seat correctly or they don't seat at all.

Fire hose thread geometry:

• Included angle: 60 degrees
• Flat at root and crest of basic thread form: $= \frac{1}{8} \times \text{pitch}$
• Height of thread: $= 0.649519 \times \text{pitch}$

Thread designation format: nominal size – threads per inch NH

Example: 2½–7.5 NH

Selected fire hose thread dimensions:

The Decision Matrix — Choosing the Right Pipe Thread System

the practitioner, two years after the flooding incident, now teaches a course at his local trade school. The first slide in his presentation is this matrix. He says it's the single most useful thing he wished he'd had on day one.

Primary Selection Matrix

Secondary Decision: Dryseal Subtype Selector

Does the assembly require metal-to-metal sealing without any sealant?
  │
  YES
  │
  ├─ Is full thread length available on both male and female?
  │     YES → NPTF (full length) ← Always the first choice
  │
  ├─ Is thread length limited by design?
  │     YES → PTF-SAE SHORT (one thread shorter)
  │              │
  │              └─ Further constrained? → PTF-SPL SHORT or PTF-SPL EXTRA SHORT
  │
  ├─ Is the internal thread in ductile soft material, economy critical?
  │     YES → NPSF (straight internal) with NPTF external
  │
  └─ Is the internal material hard/brittle with thick section, little expansion?
        YES → NPSI (straight internal, slightly larger) with PTF-SAE SHORT external

Thread Engagement Quick Reference for NPT

Assembling the Full Picture — What the practitioner's Installer Should Have Known

The joint that failed at the practitioner's plant was a ½-inch pipe connection in a high-pressure coolant circuit. The specification called for:

1⁄2-14 NPTF-1

What was installed was a standard NPT fitting assembled with PTFE tape. The nominal geometry was identical. The failure mode was invisible for six years.

Here is the critical distinction rendered as engineering fact:

The same nominal pipe size, the same taper, the same pitch — and a completely different sealing philosophy.

The thread symbol carries the engineering intent of the designer. Ignoring it doesn't mean the joint fails immediately. It means the joint fails when conditions change — higher cycling frequency, elevated temperatures, vibration, or aging sealant — and it fails in a way that looks like a random event but is actually an engineered consequence.

Putting It All to Work — A Practical Workshop

Here is a structured checklist for any new pipe thread application:

Engineering Specification Checklist

Step 1 — Identify the joint requirement:

• Pressure-tight? → Requires NPT (with sealant) or NPTF (without sealant)
• Mechanical only? → NPSM, NPSL, NPTR
• Hose application? → NH, NHR, NPSH series
• Fire suppression? → NH (NFPA) with Higbee Cut

Step 2 — Select thread series:

• Use Primary Selection Matrix above
• Confirm dimensional availability for nominal pipe size

Step 3 — Check thread length and engagement:

• Verify material wall thickness can accommodate thread length
• For NPTF: confirm full L2 engagement achievable; if not, drop to PTF-SAE SHORT with engineering review
• For special dryseal: confirm assembly conditions (Table 10 — at-limit tolerance behavior)

Step 4 — Select tap drill size:

• For taper pipe taps: use Dryseal tap drill table; ream to 3⁄4 in./ft taper
• For straight pipe threads: use NPSF/NPSI column in tap drill table
• For standard UNC/UNF: use 75% thread formula, select nearest commercial drill

Step 5 — Verify designation callout:

• Format: nominal size – threads/inch SYMBOL-class
• Example: 3⁄8-18 NPTF-1 or 1⁄2-14 NPT or G3⁄4 B
• Left-hand thread? Add LH suffix
• Plating/coating? Add BEFORE COATING / AFTER COATING with diameter limits

Step 6 — Gaging and inspection:

• NPT: ANSI/ASME B1.20.1 gages
• NPTF: ANSI B1.20.5 (inch) or ANSI B1.20.6M (metric translation)
• NH Fire Hose: NFPA 1963 gage specifications

The Universal Takeaway: Symbols Are Not Cosmetic

the practitioner's plant incident was not a failure of hardware. The physical thread — the helix of metal cut into a steel pipe — was geometrically correct. It was a failure of classification, a misread of the engineering intent encoded in a two-to-four-letter thread symbol.

This chapter covers what might seem like a narrow technical subject: the designations of pipe threads. But the underlying principle is one that governs all of engineering:

Every specification symbol carries the full engineering intent of the person who wrote it. Substituting a "close enough" alternative is not conservatism — it is the removal of a design decision without the authority to make that change.

Pipe thread systems exist because different applications require fundamentally different joint mechanics:

• Taper vs. straight — changes how the joint seals
• Metal-to-metal vs. sealant-dependent — changes the failure mode
• Full thread vs. shortened — changes the margin of assembly tolerance
• Hose coupling vs. structural joint — changes the load path entirely

Every symbol in the thread designation table — NPT, NPTF, NPSC, NPSM, PTF-SAE SHORT, NPTF-1, G, NH, NH with Higbee Cut — exists because an engineer at some point encountered the failure mode that the new designation was designed to prevent.

When you read a pipe thread callout, you are reading the compressed history of those failures.

Read it correctly.

Your Next Step

For the beginner: Pull out any pipe fitting in your workspace. Find the specification on its packaging or prints. Locate the thread symbol. Now use the designation table in Part IX to identify: Is this joint designed to seal by taper engagement with a sealant? By metal-to-metal Dryseal contact? Or is it a mechanical connection only?

Understanding which category your fitting belongs to is the single most valuable pipe-threading skill you can develop this week.

For the experienced engineer or machinist: When was the last time you reviewed a thread callout and questioned whether the specified thread type (not just the nominal size) was correct for the service conditions? Review your most pressure-critical piping system and verify that every fitting symbol matches the intended sealing mechanism.

For the designer or specifying engineer: Has your organization's standard pipe assembly procedure specified which Dryseal subtype to use for which pressure range and material combination? If not, Table 8 (Assembly Limitations for Dryseal Threads) and the Primary Selection Matrix above are your starting point for building that specification.

What's the most consequential thread substitution error you've encountered in the field — and what did it take to find it? Share your experience or question in the comments below. Engineering knowledge compounds when it's shared.

Series Navigation:

← Chapter 7: Wing Screws, Thumb Screws & Specialized Fastener Forms

→ Chapter 9: Acme, Buttress & Power Transmission Thread Systems

All dimensional data and tolerance specifications referenced in this chapter are sourced from ANSI/ASME B1.20.1-1983 (R1992), ANSI B1.20.3, ANSI B1.20.7-1991, BS 21:1973, BS 2779:1973, and NFPA Standard No. 1963-1993. Verify current editions before specifying for new designs.

The Thread That Wouldn't Seal

"The leak started as a drip. By morning, it was a problem that cost the project three weeks and a relationship with a client."

Understanding the Landscape — The Four Jobs a Pipe Thread Can Do

Before diving into geometries and standards, you need to understand the philosophical foundation of pipe thread design. Every pipe thread in existence performs one of exactly four functions:

the practitioner's mistake — and it's one of the most common mistakes in piping design — was specifying NPT threads and assuming the thread geometry alone would seal the joint. NPT threads require sealant. The design assumes it. The geometry is built around it. Without sealant, the taper creates mechanical engagement, but the spiral leak path along the thread flanks is never fully closed.

When the sealant was improperly applied or failed to fill that spiral path, the joint leaked. Not because the thread was bad. Because the thread was doing exactly what it was designed to do — and the team didn't understand that design.

The American National Standard Taper Pipe Thread (NPT) — The Workhorse of Fluid Systems

What Makes It Work

The American National Standard Taper Pipe Thread (NPT), governed by ANSI/ASME B1.20.1-1983 (R1992), is the foundation of piped systems across manufacturing, energy, and construction.

Its defining characteristic is its taper: the thread diameter increases progressively along the length of the pipe, at a rate of 3/4 inch per foot (1 in 16) measured on the diameter. The corresponding half-angle of taper with the centerline is 1 degree, 47 minutes.

The thread profile uses a 60-degree included angle in the axial plane, with the bisecting line perpendicular to the axis. The crest and root are truncated — not sharp V-threads — with the basic maximum thread height given by:

$$h = 0.8 \times p$$

where $p$ is the pitch (in inches) = $\frac{1}{n}$, and $n$ is the number of threads per inch.

The truncation at crest and root is a minimum of $0.033 \times p$ for all pitches.

The Critical Pitch Diameter Formulas

The pitch diameter system for NPT threads is defined by two reference locations:

• $E_0$ — Pitch diameter at the end (small end) of the pipe (external thread)
• $E_1$ — Pitch diameter at the gaging notch (large end of internal thread; handtight engagement plane)

The governing formulas are:

$$E_0 = D - (0.05D + 1.1) \cdot p$$

$$E_1 = E_0 + 0.0625 \cdot L_1$$

where:

• $D$ = outside diameter of pipe
• $p$ = pitch = $1/n$
• $L_1$ = length of hand-tight engagement between external and internal threads
• $L_2$ = basic length of effective external taper thread

The diameter increases per thread along the taper are:

$$\Delta d_{\text{per thread}} = \frac{0.0625}{n}$$

NPT Basic Dimensions — The Reference Table (ANSI/ASME B1.20.1-1983)

This is the cornerstone table. Every designer and machinist working with NPT threads should have it committed to memory — or at minimum, know how to read it.

Note: All dimensions are in inches. These are expressed to four or five decimal places to eliminate computational errors in gage manufacture — not to imply manufacturing precision at that level.

Thread Length: What L₁ and L₂ Actually Mean

Understanding the reference lengths prevents costly mis-specification.

    |<---- L5 --->|<-- L3 -->|<----- L1 ----->|<-- V -->|
    |             |          |                 |         |
    E5            E3         E0                E1        L4
    |             |          |                 |         |
    |----- L2 (Effective External Thread) ----->|

    Taper: 1 in 16 (measured on diameter)
    Thread angle: 60°
    Half-taper angle: 1°47'

• $L_1$ — Handtight engagement length. This is the length over which a hand-assembled joint engages without tools. Gaging controls this length.
• $L_2$ — Full effective external thread length. Includes approximately two imperfect-crested threads near the end of the pipe.
• $L_3$ / $E_3$ — Wrench makeup length/diameter for internal thread vanish (three threads for sizes ≤ 2 in.; two threads for larger sizes).
• $L_4$ — Overall external thread length.
• $L_5 = L_2 - 2p$ — Length to the plane where thread form becomes imperfect at the crest.

The wrench makeup (driven engagement beyond handtight) is the critical sealing zone. It forces the taper flanks into contact and compresses the sealant into any remaining spiral leak path.

Limits on Crest and Root — NPT (ANSI/ASME B1.20.1-1983)

The thread form is not a theoretical sharp V. Crest and root are truncated within controlled limits. Both tool wear and controlled truncation affect the width of the flat at crest and root.