ASME B1.20.3

Covers — dimensions, tolerances, and gauging for dryseal inch pipe threads:

Size range — 1/16″ through 2″ NPS. Sizes above 2″ NPS are generally not available in NPTF because the crest/root contact forces at large diameters are impractical with standard torque.

Does NOT cover (exclusions) — the following are governed elsewhere:

• General-purpose (sealant-required) taper pipe threads → ASME B1.20.1 (NPT)
• ISO tapered/parallel pipe threads → ISO 7-1 (R/Rc) / ISO 228-1 (G)
• Threaded fitting dimensions and pressure ratings → ASME B16.11 (forged), ASME B16.3 (malleable iron)
• Hydraulic port threads (O-ring face seal or 37° flare) → SAE J514 / SAE J1926
• Where threaded joints are permitted in piping systems → the governing piping code (e.g., ASME B31.3)

A genuine dryseal joint requires both the external and internal thread to be NPTF — mating NPTF with NPT requires a sealant and is not a dryseal connection.

B1.20.3 establishes minimum requirements for NPTF dryseal threads that differ from NPT primarily in crest/root truncation control:

Thread Geometry (identical taper to NPT)

• Thread angle — 60° included angle (same as NPT).
• Taper — ¾ in/ft on diameter (same as NPT).
• Crest and root — controlled flat width (narrower tolerance band than NPT). The crest/root must contact before the flanks fully load — this is what creates the metal-to-metal seal path that eliminates the helical leak channel.

Critical Difference: Crest/Root vs. NPT

Thread Size Range and TPI

NPTF uses the same TPI as NPT for all sizes (27 TPI at 1/8″, 18 TPI at 1/4″ and 3/8″, 14 TPI at 1/2″ and 3/4″, 11½ TPI at 1″ through 2″). The dimensional geometry is the same — only the crest/root truncation tolerance is tighter.

Gauging System (more stringent than NPT)

NPTF requires an additional crest-check gauging step beyond NPT's L1/L2/L3 system:

• L1 ring and plug gauges — same function as NPT: verify pitch diameter at hand-tight engagement plane.
• Crest-check (W) ring gauge — applied to the external thread; measures the minor diameter to verify crest truncation is within the tight NPTF band. This gauge confirms the dryseal capability.
• Crest-check (W) plug gauge — applied to the internal thread; verifies root diameter to confirm internal crest truncation.
• L2 ring gauge — verifies effective thread length.

Designation Format

• Taper dryseal → ½-14 NPTF (same NPS/TPI as NPT, suffix changes to NPTF)
• Straight internal dryseal → ½-14 NPSF or ½-14 NPSI

How NPTF Achieves a Dryseal

ASME B1.20.3's NPTF thread is geometrically similar to NPT — same 60° angle, same ¾ in/ft taper, same pitch — but the crest and root truncations are held to a far tighter tolerance. This is the only difference, and it changes the sealing physics completely.

In NPT, when two threads are mated, the 60° flanks bear the load while a small but definite helical clearance remains between the crests and roots — that gap is the leak path that PTFE tape or pipe dope must fill.

In NPTF, the tighter crest/root truncation is calculated so that the crests of one thread contact and crush the roots of the mating thread before the flanks fully load. This closes the helical gap without any sealant — the "dryseal."

NPTF Thread Variants

When to Use NPTF Instead of NPT

NPTF vs. NPT — Key Comparison

NPTF and NPT threads can be assembled together but the result is not a dryseal — apply a sealant for any NPT × NPTF pairing.