In the field of pipe fittings, the term typically refers to components that join two or more pipes together—either through threading or a slip-fit mechanism. While metal pipes usually feature pipe threads, plastic pipes may be connected using either method. A threaded connection involves screw-like ends that allow components to be fastened tightly together.
For example, the designation “½–14 NPT” refers to a pipe thread with a nominal internal diameter of ½ inch and 14 threads per inch, in line with the National Pipe Taper (NPT) standards. If the thread type is marked with “LH,” it indicates a left-handed thread. Common pipe thread standards used in the United States include:
- NPT: American Standard Taper Pipe Thread
- NPSC: American Standard Straight Coupling Pipe Thread
- NPTR: American Standard Taper Railing Pipe Thread
- NPSM: American Standard Straight Mechanical Pipe Thread
- NPSL: American Standard Straight Locknut Pipe Thread
The term “taper” is important when discussing pipe threads. Unlike bolts or screws, many pipe threads are tapered to help create both a secure mechanical fit and a leak-resistant seal. This tapering means the threads become progressively shallower further from the pipe’s end. The typical taper is 1/16 inch per inch.
Due to this taper, a pipe will only screw into a fitting partway before it tightens firmly. Two measurements define this: effective thread, the standard contact area, and engagement, which refers to how far the pipe can be hand-tightened before needing tools. These values help professionals understand how many turns are needed for a proper fit.
Thread Types Found in Pipe Fittings
Various pipe thread types are used in different fittings, including:
- Right- and Left-Handed Threads
Most threads are right-handed—tightened clockwise and loosened counterclockwise. Left-handed threads are the reverse, though they’re less common. - Male Threads
These have external threads on the pipe and typically use tapered pipe threads such as NPT or BSPT for a tight seal, often without additional gaskets. - Female Threads
Found inside fittings, these threads accept the male counterpart. A typical example is the female iron pipe thread, designed to securely join with a male pipe using a tapered configuration. - Male Straight Thread
Threads like UNC, UNF, or ASME types that do not taper fall into this category. - Female Straight Thread
These are internal, straight threads, matching with male straight counterparts under UNC, UNF, or ASME standards.
Other connection styles include:
- Plain End: Inserted into the expanded or flared end of a connecting pipe.
- Bell / Socket / Flare End: An enlarged opening that accommodates the pipe.
- Flange: A flat interface that connects pipes using bolts or welds—available in circular or square designs.
- Compression Fittings: Use a nut and ferrule to grip the pipe tightly.
- Clamp End: Designed for securing to a pipe or other fitting.
- Barb / Rib: Used to connect flexible hoses or soft piping, often with a clamp.
- Groove: Allows for the inclusion of sealing elements like o-rings or gaskets.
Understanding these pipe thread types and standards ensures the right selection for leak-proof, secure, and durable piping systems, especially when using female iron pipe thread fittings.
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