Page 613 - DixonCatalog20092010

611 DPL410 877.963.4966 • www.dixonvalve.com
T
Pipe Threads
Pipe threads are either tapered or straight (parallel). The two styles are not compatible except for NPSL (National Pipe Straight
Locknut), which is compatible with NPT.
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 to form a seal.
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. When used together, the threads and sealing method provide a better overall seal
than tapered threads. Straight pipe threads include NPSM (National Pipe Straight Mechanical),
NPSL (National Pipe Straight Locknut) and NPSH (National Pipe Straight Hose) conform to
SAE (Society of Automotive Engineers specifications. Sizing and pitches differ from the NPT
threads
Less common straight threads are GHT (Garden Hose) and NST (fire hose coupling).
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). The two styles are not compatible.
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"
1/8"
1/4"
3/8"
1/2"
3/4"
1"
1-1/4"
1-1/2"
2"
2-1/2"
3"
3-1/2"
4"
5"
6"
8"
27
27
18
18
14
14
11½
11½
11½
11½
8
8
8
8
8
8
8
---
28
19
19
14
14
11
11
11
11
11
11
11
11
11
11
11
NPT vs. BSP (British Standard Pipe)
tapered threads
straight threads
BSPT/BSPP threads
NPT threads