Pipe, Dryseal and Special-Purpose Threads: The Dryseal Thread Selection Framework

The Dryseal Thread Selection Framework

START: Does your joint require a pressure-tight seal?
│
├── NO ─→ Consider NPT with sealant, or BS 2779 (G series)
│          for mechanical fastening applications
│
└── YES ─→ Can both components carry full NPTF thread length?
           │
           ├── YES ─→ Specify NPTF Class 2 (both components)
           │          → Best sealing, maximum reliability
           │
           └── NO ─→ Is the limitation on space or material thickness?
                     │
                     ├── SPACE limited ─→ Use PTF-SAE SHORT external
                     │                   Pair with NPSI internal
                     │
                     ├── MATERIAL is ductile (Al, brass, soft steel)
                     │   ─→ Use NPSF internal + NPTF external
                     │
                     ├── MATERIAL is hard/brittle or thin-walled
                     │   ─→ Use NPSI internal + NPTF external
                     │
                     └── EXTREME space/material constraint?
                         ─→ Consider PTF-SPL SHORT or PTF-SPL EXTRA SHORT
                            (consult assembly tolerance limits)

Quick-Reference: Dryseal Thread Type Summary

Quick-Reference: Thread Designation Examples

The Formulas Behind the Standard

The core pitch diameter relationships in Dryseal threads follow the same formulas as NPT, because the thread form is the same. The key variable is truncation control.

Pitch diameter at end of pipe:

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

Pitch diameter at hand-tight engagement (gaging notch):

$$E_1 = E_0 + L_1 \times \frac{0.0625}{n}$$

Where:

• $D$ = outside diameter of pipe
• $p$ = pitch ($= 1/n$)
• $n$ = threads per inch
• $L_1$ = hand-tight engagement length
• $0.0625/n$ = increase in pitch diameter per thread (due to taper)

Thread height for Dryseal (truncated form):

$$h = 0.8p$$

Minimum truncation at root (the value that ensures crest contact before flank contact):

$$f_{\text{root, min}} = 0.078p \text{ to } 0.094p \text{ (varies by pitch)}$$

These formulas are the mathematical foundation of every tolerance table in ANSI B1.20.3. Understanding them lets you compute thread geometry for non-standard sizes, verify gage dimensions, and troubleshoot out-of-tolerance conditions analytically.

The Lesson That Doesn't Need a Sealant

the practitioner's refrigeration system taught him something that most engineers only learn from a failure: a joint designed to seal itself is more reliable than a joint that relies on something added to it.

Dryseal pipe threads embody that principle at the level of geometry. The seal is in the thread form. The wider root crushes the narrower crest. Metal contacts metal at both major and minor diameters before the flanks can carry load. The spiral leak path closes. The joint is tight — not because you wrapped it in tape, but because the engineers who designed ANSI B1.20.3 understood that true reliability lives in the geometry.

For the beginner: this is your introduction to a thread standard that belongs in every fluid system you design.

For the expert: this is your complete reference — truncation tables, assembly matrices, special series, tap drills, and designation conventions, all in one place.

For the client evaluating a supplier: ask whether they can hold NPTF Class 2 truncation limits. The answer tells you more about their threading capability than almost any other single question.

Your Next Step

The next time you specify a pipe connection — before you reach for the PTFE tape dispenser, before you write "NPT with sealant" on the drawing — ask one question:

"Can this joint be designed to seal itself?"

If the application is a fluid system subject to vibration, thermal cycling, or pressure transients — and the answer is anything other than "yes, and here's why I chose NPTF Class 2 or PTF-SAE SHORT" — you have a design improvement waiting to happen.

Leave a comment below — what application prompted you to investigate Dryseal threads for the first time? Was it a leaking refrigerant line, a hydraulic system under vibration, a fuel fitting in an engine bay? The story behind the specification is often the most useful learning for the engineers who come after you.

Reference standards: ANSI B1.20.3-1976 (R1998), ANSI B1.20.4, ANSI B1.20.5, ANSI B1.20.6M, ANSI/ASME B1.20.1-1983 (R1992), BS 2779:1973, BS 21:1973

This post is part of a serialized technical reference series covering major engineering domains: manufacturing processes, machine elements, fasteners, threads and threading, gears, splines, cams, and measurement systems.