Thread Standards

Reference data and engineering information about thread standards for standard organizations applications.

Overview

Engineering reference data for Thread Standards in standard organizations.

$ISO \; 9001:2015$

Quality management systems.

$ASTM \; E8$

Standard test methods for tension testing.

$ANSI/ASME \; B16.5$

Pipe flanges and flanged fittings.

Symbol	Description	Unit
$ISO$	International Organization for Standardization	—
$ASTM$	American Society for Testing and Materials	—
$ANSI$	American National Standards Institute	—

The following sections categorize major thread standards by type and application, with applicable governing specifications.

Designation	Full Name	Primary Application	Applicable Standards
NPSC	American Standard Straight Pipe Threads in Pipe Couplings	General-purpose coupling connections	ANSI B1.20.1, FED-STD-H28/7
NPSF	Dryseal American Standard Fuel Internal Straight Pipe Threads	Fuel system connections requiring dry seal	SAE J476, ANSI B1.20.3, FED-STD-H28/8
NPSI	Dryseal American Intermediate Internal Straight Pipe Threads	Intermediate pressure dry-seal applications	SAE J476, ANSI B1.20.3, FED-STD-H28/8
NPSM	American Standard Straight Pipe Threads for Free-Fitting Mechanical Joints	Swivel nuts and adapters (fits freely over NPTF)	ANSI B1.20.1, FED-STD-H28/7

Designation	Full Name	Primary Application	Applicable Standards
ANPT	Aeronautical National Taper Pipe Threads	Aerospace hydraulic and pneumatic systems	MIL-P-7105
NPT	American Standard Taper Pipe Threads for General Use	General-purpose pressure-tight connections	ANSI B1.20.1, FED-STD-H28/7
NPTF	Dryseal American Standard Taper Pipe Threads	Dry-seal applications (no sealant required)	SAE J476, ANSI B1.20.3, FED-STD-H28/8
PTF-SAE Short	Dryseal SAE Short Taper Pipe Threads	Low-pressure pneumatic and fuel applications	ANSI B1.20.3
PTF-SPL	Dryseal Special Short Taper Pipe Threads	Pneumatic components with size limitations	ANSI B1.20.3
AWWA	American Water Works Association	Underground service line valves and fittings	ANSI/AWWA C800-05

Designation	Full Name	Pitch Series	Applicable Standards
UN	Unified Constant Pitch Threads	4, 6, 8, 12, 16, 20, 28, 32 TPI	ASME B1.1, FED-STD-H28/2
UNC	Unified Coarse Threads	Standard coarse	ASME B1.1, FED-STD-H28/2
UNF	Unified Fine Threads	Standard fine	ASME B1.1, FED-STD-H28/2
UNEF	Unified Extra Fine Threads	Extra fine	ASME B1.1, FED-STD-H28/2
UNS	Unified Special Pitch Threads	Non-standard/special	ASME B1.1
UNJ	Unified Controlled Root Radius Threads	Controlled root radius for fatigue resistance	ANSI B1.15, FED-STD-H28/4

Designation	Full Name	Primary Application	Applicable Standards
M	Metric Screw Threads (M Profile)	General-purpose metric fasteners	ISO 261, ASME B1.13M, FED-STD-H28/21
Keg	Metric Taper Threads	Taper connections (primarily used in Germany)	DIN 158-1, DIN 158-2

Designation	Full Name	Thread Form	Applicable Standards
R (BSPT)	British Standard Taper Pipe Threads, External	Taper external	BS 21 (withdrawn), ISO 7
Rc (BSPT)	British Standard Taper Pipe Threads, Internal	Taper internal	BS 21 (withdrawn), ISO 7
Rp / G (BSPP)	British Standard Pipe Parallel Threads	Parallel	BS 21 (withdrawn), ISO 228

When selecting thread standards for an application, consider:

Pressure-tight seal method: NPT/NPTF/BSPT threads seal on the threads themselves; NPSM/BSPP require O-rings or gaskets
Dry vs. wet seal: Dryseal standards (NPTF, NPSF, NPSI) do not require sealant tape or compound
Application environment: Aerospace applications require ANPT per MIL-P-7105; fuel systems typically specify NPTF
Material compatibility: Metric (M) and Unified (UN/UNC/UNF) threads are for mechanical fastening, not fluid sealing
Dimensional system: Inch-based (ANSI/ASME/SAE) vs. metric (ISO/DIN) based on regional standards