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BSPP, BSPT and NPT thread

Threaded connection plays an important role in pipe connection and seal where conveys liquids, gases and steam. Till today, in threaded connection world it is governed by 2 kinds — British Standard Pipe Thread (BSP) and National pipe tapered (NPT).
BSP was created by British engineer Joseph Whitworth in 1841 that are widely used in Europe, Africa and most Asian coutries.
NPT was invented several years later in 1864 by American engineer William Sellers which are applicable in American countries.

BSP

BSP includes BSPP (British Standard Pipe Parallel) and BSPT (British Standard Pipe Taper).
BSPP
British Standard Pipe Parallel Thread (BSPP) has been adopted internationally as a standard scale used in plumbing fittings, and it is defined by BS EN 10226-1 and ISO 7-1, or ISO 228-1 (GB/T 7306.1, GB/T 7307).
BSPP thread has Whitworth thread form (rounded peaks and valleys) with thread angle 55°.
It is designated for both pressure-tight connection (Rp x R1) or not pressure-tight connection (G x G).
BSPP
BSPP (Rp) thread dimension
Pipe size Thread number in 25.4mm Thread pitch (P) Thread height (h) Diameter at gauge plane Tolerance on position of gauge plane on internal thread *
Major gauge diameter (d=D) Pitch (d₂=D₂) Minor (d₁=D₁) Tolerance ±T₂/2
(inch) (mm) (no.) (mm) (mm) (mm) (mm) (mm) (inch) Turns of thread
1/4" 13.5 19 1.337 0.856 13.157 12.301 11.445 1.7 1 1/4
3/8" 17.2 19 1.337 0.856 16.662 15.806 14.950 1.7 1 1/4
1/2" 21.3 14 1.814 1.162 20.955 19.793 18.631 2.3 1 1/4
3/4" 26.9 14 1.814 1.162 26.441 25.279 24.117 2.3 1 1/4
1" 33.7 11 2.309 1.479 33.249 31.77 30.291 2.9 1 1/4
1-1/4" 42.4 11 2.309 1.479 41.910 40.431 38.952 2.9 1 1/4
1-1/2" 48.3 11 2.309 1.479 47.803 46.324 44.845 2.9 1 1/4
2" 60.3 11 2.309 1.479 59.614 58.135 56.656 2.9 1 1/4
2-1/2" 76.1 11 2.309 1.479 75.184 73.705 72.226 3.5 1 1/2
3" 88.9 11 2.309 1.479 87.884 86.405 84.926 3.5 1 1/2
4" 114.3 11 2.309 1.479 113.030 111.551 110.072 3.5 1 1/2
5" 139.7 11 2.309 1.479 138.430 136.951 135.472 3.5 1 1/2
6" 165.1 11 2.309 1.479 163.83 162.351 160.872 3.5 1 1/2
Note: * For more dimensions of thread please refer to BS EN 10226-1 table 1 and ISO 7-1 table 1.
BSPP (G) thread dimension
Pipe size Thread number in 25.4mm Thread pitch (P) Thread height (h) Diameter at gauge plane Tolerances on pitch diameter
Major gauge diameter (d=D) Pitch (d₂=D₂) Minor (d₁=D₁) Internal thread TD2*
Lower deviation Upper deviation
(inch) (mm) (no.) (mm) (mm) (mm) (mm) (mm) (mm) (mm)
1/4" 13.5 19 1.337 0.856 13.157 12.301 11.445 0 + 0.125
3/8" 17.2 19 1.337 0.856 16.662 15.806 14.950 0 + 0.125
1/2" 21.3 14 1.814 1.162 20.955 19.793 18.631 0 + 0.142
3/4" 26.9 14 1.814 1.162 26.441 25.279 24.117 0 + 0.142
1" 33.7 11 2.309 1.479 33.249 31.77 30.291 0 + 0.180
1-1/4" 42.4 11 2.309 1.479 41.910 40.431 38.952 0 + 0.180
1-1/2" 48.3 11 2.309 1.479 47.803 46.324 44.845 0 + 0.180
2" 60.3 11 2.309 1.479 59.614 58.135 56.656 0 + 0.180
2-1/2" 76.1 11 2.309 1.479 75.184 73.705 72.226 0 + 0.217
3" 88.9 11 2.309 1.479 87.884 86.405 84.926 0 + 0.217
4" 114.3 11 2.309 1.479 113.030 111.551 110.072 0 + 0.217
5" 139.7 11 2.309 1.479 138.430 136.951 135.472 0 + 0.217
6" 165.1 11 2.309 1.479 163.83 162.351 160.872 0 + 0.217
Note:
D = d; major diameter of the internal thread
D₁ = D - 1.280 654 P = d₁; minor diameter of the internal thread
D₂ = D - 0.640 327 P = d₂; pitch diameter of the internal thread
d = Major diameter of the internal thread
d₁ = d - 1.280 654 P = d₁; minor diameter of the external thread
d₂ = d - 0.640 327 P = d₂; pitch diameter of the external thread
H = Height of the fundamental triangle of the thread
h = Height of the thread profile with rounded crests and roots
P = Pitch
r = Radius of rounded crests and roots
* For more dimensions of thread please refer to ISO 228-1 table 1.
BSPT
British Standard Pipe Taper Thread (BSPT) has been adopted internationally as a standard scale used in plumbing fittings, and it is defined by BS EN 10226-1, BS EN 10226-2, and ISO 7-1 (GB/T 7306.1, GB/T 7306.2). BSPT thread has Whitworth thread form (rounded peaks and valleys) with thread angle 55° and tapered thread 1°47’.
BSPT
BSPT thread dimension
Pipe size Thread number
in 25.4mm		 Thread pitch
(P)		 Thread height
(h)		 Diameter at gauge plane *
Major gauge diameter
(d=D)		 Pitch
(d2=D2)		 Minor
(d1=D1)
(inch) (mm) (no.) (mm) (mm) (mm) (mm) (mm)
1/4" 13.5 19 1.337 0.856 13.157 12.301 11.445
3/8" 17.2 19 1.337 0.856 16.662 15.806 14.95
1/2" 21.3 14 1.814 1.162 20.955 19.793 18.631
3/4" 26.9 14 1.814 1.162 26.441 25.279 24.117
1" 33.7 11 2.309 1.479 33.249 31.770 30.291
1-1/4" 42.4 11 2.309 1.479 41.910 40.431 38.952
1-1/2" 48.3 11 2.309 1.479 47.803 46.324 44.845
2" 60.3 11 2.309 1.479 59.614 58.135 56.656
2-1/2" 76.1 11 2.309 1.479 75.184 73.705 72.226
3" 88.9 11 2.309 1.479 87.884 86.405 84.926
4" 114.3 11 2.309 1.479 113.030 111.551 110.072
5" 139.7 11 2.309 1.479 138.430 136.951 135.472
6" 165.1 11 2.309 1.479 163.830 162.351 160.872
Note:
D = d; major diameter of the internal thread
D₁ = D - 1.280 654 P = d₁; minor diameter of the internal thread
D₂ = D - 0.640 327 P = d₂; pitch diameter of the internal thread
d = Major diameter of the internal thread
d₁ = d - 1.280 654 P = d₁; minor diameter of the external thread
d₂ = d - 0.640 327 P = d₂; pitch diameter of the external thread
H = Height of the fundamental triangle of the thread
h = Height of the thread profile with rounded crests and roots
P = Pitch
r = Radius of rounded crests and roots
* For more dimensions of thread please refer to BS EN 10226-1 table 1, BS EN 10226-2 table 1 and ISO 7-1 table 1.

NPT

National Pipe Thread Taper (NPT) is a US standard for tapered threads used to join pipes and fittings, and it is defined by ANSI/ASME standard B1.20.1 (GB/T 12716).
It has a Sellers thread form (flattened peaks and valleys) with thread angle 60° and tapered thread 1°47’.
NPT thread dimension
Pipe size Thread number Thread pitch Thread height Hand-tight engagement *
D No. P h Length Threads Diameter
(inch) (mm) (no. / inch) (inch) inch (L1, inch) (no.) (E1, inch)
1/4" 13.7 18 0.05556 0.04444 0.2278 4.10 0.49163
3/8" 17.1 18 0.05556 0.04444 0.240 4.32 0.62701
1/2" 21.3 14 0.07143 0.05714 0.320 4.48 0.77843
3/4" 26.7 14 0.07143 0.05714 0.339 4.75 0.98887
1" 33.4 11.5 0.08696 0.06957 0.400 4.60 1.23863
1-1/4" 42.2 11.5 0.08696 0.06957 0.420 4.83 1.58338
1-1/2" 48.3 11.5 0.08696 0.06957 0.402 4.83 1.82234
2" 60.3 11.5 0.08696 0.06957 0.436 5.01 2.29627
2-1/2" 73.0 8 0.12500 0.10000 0.682 5.46 2.76215
3" 88.9 8 0.12500 0.10000 0.766 6.13 3.3885
4" 114.3 8 0.12500 0.10000 0.844 6.75 4.38712
5" 141.3 8 0.12500 0.10000 0.937 0.94 5.44929
6" 168.3 8 0.12500 0.10000 0.958 7.66 6.50597
Note:
L1 = length of hand-tight engagement between internal and external threads.
E1 = basic pitch diameter at plane of hand-tight engagement length (L1), external thread, and large end of internal thread.
L2 = length of effective thread, external thread. Equal to L1 plus wrench make-up threads.
E2 = basic pitch diameter at plane of effective thread length (L2), external thread.
E3 = basic pitch diameter at plane of wrench-tight engagement.
* For more dimensions of thread please refer to ASME B1.20.1 table 2.

Threads connection

For BSPP pressure-tight connection, the threads are parallel internal and taper external and parallel (Rp x R1).
For BSPP not pressure-tight connection, the threads are parallel internal and parallel external (G x G).
If assemblies with such threads must be made pressure-tight, this should be effected by compressing two tightening surfaces outside the threads, and by interposing an appropriate seal.(Such as adding O-ring).
For BSPT connection, the threads are taper internal and taper external (Female BSPT x Male BSPT).
For NPT connection, the threads are taper internal and taper external (Female NPT x Male NPT).
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