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We sell a lot of plumbing/pneumatic
products which are sized to engineering standards that are
commonly used in engineering, however for people not familiar
with engineering, size identification can be difficult, with
the result of purchases being returned as customers have
purchased the wrong size. The aim of this page is to remove
any doubt when ordering plumbing and pneumatic fittings.
I'm not going to go in to great detail on
types of threads, cutting angles etc. my aim is to remove
doubt when sizing of products.
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Plumbing pipe (solid pipe, not
flexible or copper) size is NOT calculated on the outside
diameter as show by 'B' below
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Plumbing pipe size is calculated on
its inside diameter, and this dimension remains imperial,
for example 1/2", 3/4" etc
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Plumbing fitting sizes are known by
their size and the type of thread. All water plumbing
threads are known as BSP (British Standard Pipe Thread)
occasionally you'll come across BSPT which indicates the
thread is tapered ('T'), rather than parallel. So you'll get 1/2"BSP, 3/4"BSP etc.
The image below represents a cross section of a pipe with threads on the end.
By measuring your pipe thread
outside diameter ('B') and relating your actual measurement to
the closest dimension in either the centre column or the right
hand column of the table below, this will give you the thread
size in the left hand column
|
Pipe & Thread Size |
'B' Outside Diameter |
|
Inch |
mm |
|
1/8" |
0.3830 |
9.728 |
|
1/4" |
0.5180 |
13.157 |
|
3/8" |
0.6560 |
16.662 |
|
1/2" |
0.8250 |
20.955 |
|
5/8" |
0.9020 |
22.911 |
|
3/4" |
1.0410 |
26.441 |
|
7/8" |
1.1819 |
30.020 |
|
1" |
1.3090 |
33.249 |
|
1 1/4" |
1.6500 |
41.910 |
|
1 1/2" |
1.8820 |
47.803 |
|
2" |
2.3470 |
59.614 |
|
2 1/2" |
2.9600 |
75.184 |
|
3" |
3.460 |
87.884 |
|
4" |
4.45 |
113.030 |
The figures in the left hand
column will give you a rough idea of the inside
diameter, but shouldn't be relied up on, as the pipe wall
thickness do vary considerably.
The above information is good
for most water pipes, however pneumatic air lines used for air
suspension systems is different, as these are 'sized' by their
outside diameter. The American kits generally use 1/4" outside
diameter air lines, where as everyone else uses metric!
Dunlop's original air lines are 5mm, however if supplied with
a basic gauge kit, the air lines can be either 4mm or 6mm. The
threads used on either the American or the Dunlop kits are
mostly 1/8"BSP and 1/4"BSP, however some threads on the Dunlop
fittings can be M5 !
Remember, if in doubt, ASK
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