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NPT or IPT pipe threads are tapered, Compression Fitting Threads are Parallel

Our illustration, adapted from Industrial Specialties Mfg., shows the key difference between tapered threads cut onto standard pipe fittings (NPT or IPT) and parallel-threads cut onto flare or compression fittings (IPT).

Thanks to Industrial Specialties for permission to adapt this image.

• ISM, Industrial Specialties Mfg. & IS Med Specialties 4091 S. Eliot St., Englewood, CO 80110-4396 Toll Free: 800.781.8487 original source: https://www.industrialspec.com/about-us/blog/detail/tapered-pipe-threads-standards-intro

[Click to enlarge any image]

NPT (National Pipe Threads) also abbreviated as IPT (Iron Pipe Threads) are tapered so that as you tighten NPT male and femal fittings the threads wedge themselves ever more-tightly together. That taper and wedging combined with proper use of a pipe thread sealant gives a leak-free joint.

That's true even when the male or female fitting is comprised of the threaded end of a pipe.

On the other hand compression fittings and flare fittings use threads that are cut in parallel, without taper.

Those fittings, used with flare and compression tubing, depend not on the pipe wall thickness but on a mechanical seal between a flared tubing end against a tapered flare fitting surface, or a compressed brass ring that is squeezed against the tubing exterior wall and also seals against a tapered surface inside of a compression fitting.

Summary of Plumbing Fitting Thread Types

Above: NPT threaded brass pipe nipples. Take a close look and on these short fittings you can see that the thread cut is tapered. [Click to enlarge any image]

• NPT - National Pipe Thread / NPT (Taper) is the American standard thread pattern for pipe connections and has been in use in the U.S. since the mid 1800's.
  
  NPT threads are cut at cut at an angle of about 60° along the length of the fitting, and the actual individual threads are cut at an angle of about 1° 47’ from the centerline of the fitting.
  
  As a male and female NPT fitting are tightened against this taper the connection is increasingly tight. However because of allowances and variations in the precise thread cuts it's necessary to use a sealant such as teflon tape or pipe thread sealant to assure a leak-free connection.
  
  NPT fittings are sub-divided into female fittings: FPT, FNPT, NPT(F) and male fittings: MPT, MNPT, NPT(M) Some sources refer to NPT as IPT (Iron Pipe Thread).
  
  IPT (Iron Pipe Thread) is equivalent to NPT; Some sources also refer to IPS (Iron Pipe Straight); using IPS risk confusion with NPS listed below.

Below: Standard 1/2" Compression-threaded (parallel-thread) nut and sleeve.

• Parallel Thread / Straight Thread (no Taper) - NPS - National Pipe Straight. To seal two fittings cut with parallel threads the fittings will rely on a gasket, O-ring, or on the mating of soft copper or brass flanged or tapered surfaces between the two fittings.
  
  Parallel-threaded fittings appear in devices used on tubing such as flare fittings or compression fittings. That's because tubing is too thin (compared with iron or brass pipe) to form a threaded mechanical joint.
  

Both of these threaded fitting types listed above are used in North America, including Canada, the U.S. and Mexico.

Above: here is an adapter that permits joining NPT threaded pipe fittings to a male Compression-threaded fitting - in this case used on a tankless water heater described

at RHEEM TANKLESS WATER HEATER LEAK REPAIR

Below: brass compression fittings showing a compression sleeve or ring and compression ferrule (or "nut") both of which have tapered, mating faces but whose threads are cut parallel to the axis of the fitting. Adapted from Ace Hardware - acehardware.com

Below: the tapered end of a male flare fitting,

and a sketch from COPPER FLARE CONNECTIONS showing how flare fittings join flexible copper tubing.

...

• BSPT - British Standard Pipe Threads are cut at a 55 degree angle and have more-rounded thread surfaces than their American friends.
  
  Watch out: for that reason BSPT threaded fittings are not compatible with NPT nor NPS.

Other pipe thread patterns and standards

• Society of Automotive Engineers (SAE)/MS) Internal Straight Thread Boss (parallel) - usually sealed by O-rings.
• International Standards Organization (ISO) 7/1 (British Standard Pipe Tapered) - also known as Whitworth, and found on my Norton 850 Commando motorcycle. Cut on a 55 degree angle these threads cannot be joined with an American 60 degree angle thread found on NPT.
• International Standard Organization (ISO) 228/1 British Standard Pipe Parallel

If you are not sure what threads were cut into a pipe, fitting, nut or bolt, you can use a simple thread gauge like the one shown below.

Also see

• COPPER PIPING in BUILDINGS
• GALVANIZED STEEL WATER PIPING

then the diagnosis and repair of a leak traced to use of an improper parallel threaded compression fitting found at

• RHEEM TANKLESS WATER HEATER LEAK REPAIR
  

...

Continue reading  at or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• DRAIN & SEWER PIPING - home
• GALVANIZED STEEL WATER PIPING
• GALVANIZED IRON PIPE INSTALLATION
• GAS PIPING, VALVES, CONTROLS - home
• PIPING in BUILDINGS, CLOGS, LEAKS, TYPES - home
• PLASTIC PIPING ABS CPVC PB PEX PPR PVC - home
• PLUMBING SYSTEM INSPECT DIAGNOSE REPAIR - home
  
• PLUMBING VENT DEFECTS & NOISES - home
• RHEEM TANKLESS WATER HEATER LEAK REPAIR
• SUPPLY PIPING - home
• BRASS WATER PIPES
• BRASS WATER PIPE INSTALLATION
• COMPRESSION FITTINGS
• COMPRESSION THREAD vs PIPE THREAD
• COPPER PIPING in BUILDINGS
• GALVANIZED STEEL WATER PIPING
• GALVANIZED IRON PIPE INSTALLATION
• WATER HEATER LEAK REPAIR
• WATER SUPPLY PIPE DIAMETER vs FLOW

For water pressure tank age or water storage tank age, see WATER TANK AGE DECODER

Suggested citation for this web page

COMPRESSION THREAD vs PIPE THREAD at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to WATER HEATERS

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