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Pipe Threads
Pipe threads are either tapered or straight (parallel). The two styles may or may not be compatible. Refer to thread information
chart on page 939.
Tapered threads
Tapered threads are the most common type of thread available. As the name implies, they
have a slight taper. When mated together and tightened, the threads compress and may form a
seal. Usually a thread sealant is required. The mating threads both hold the fitting in place and
seal the connection. The most widely used pipe threads in North America are NPT (National
Pipe Taper). Some confusion may result from the use of NPT, FPT, and MPT in describing
threads. Both FPT and MPT are NPT threads, with FPT meaning female threads (internal) and
MPT meaning male threads (external).
NPTF (Dryseal) threads are modified NPT threads, which are less likely to leak without a
sealant. For a leak-free seal, we recommend using a sealant compound or PTFE tape. You can
use NPTF threads with NPT threads, but you'll lose some of the leak-free characteristics.
Straight threads
Straight (parallel) threads are used for mechanical joining. They serve one purpose - to hold a
fitting in place. As a result, an O-ring (elastomer), hard metal seal or a soft seat seal is required.
Straight pipe threads include NPSM (National Pipe Straight Mechanical), and NPSH (National
Pipe Straight Hose). Sizing and pitches may differ from the NPT threads
Less common straight threads are GHT (Garden Hose) and NST (fire hose coupling).
NPT vs. BSP (British Standard Pipe)
tapered threads
straight threads
BSPT/BSPP threads
NPT threads
BSP threads are common in many countries outside the United States. BSP consists of
two types of threads - BSPT (British Standard Pipe Taper) and BSPP (British Standard Pipe
Parallel).
BSPT threads have a slight taper similar to NPT. BSPP threads are straight (parallel) threads
and have the same thread angle, shape and threads per inch (pitch) as BSPT threads.
BSPT
and BSPP threads should not be substituted for NPT threads
.
NPT and BSPT/BSPP threads have different angles, shape, and (in most cases) threads per
inch (pitch). The thread angle is 60° for NPT threads; 55° for BSPT/BSPP threads. NPT threads
are flattened at the peaks and valleys, while BSPT/BSPP threads are rounded.
Nominal
Pipe Size
Threads per inch
NPT
BSPT / BSPP
1/16"
27
---
1/8"
27
28
1/4"
18
19
3/8"
18
19
1/2"
14
14
3/4"
14
14
1"
11½
11
1-1/4"
11½
11
1-1/2"
11½
11
2"
11½
11
2-1/2"
8
11
3"
8
11
3-1/2"
8
11
4"
8
11
5"
8
11
6"
8
11
8"
8
11
Thread Information