1/2" BSP isn't half an inch?!

I just bought a self bleeding valve to fit into a half inch thread. What arrived was 3/4". It's sold as 1/2" BSP, and according to this page, 1/2" BSP is meant to be 3/4". WTF?
https://en.wikipedia.org/wiki/British_Standard_Pipe#Pipe_thread_sizes
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On 3/14/2017 5:48 PM, James Wilkinson Sword wrote:

It is pretty straightforward
http://www.bspt.co.uk/
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That makes absolutely no sense at all.
From your link: "The size was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended."
I've bought something stating it's a 1/2" BSP, yet the outer diameter of its male thread is 3/4", 50% bigger. How can the device I purchased be intended to fit into a steel tube of an internal diameter 2/3rds of the width of the device?
Where is this 1/2" measurement taken in relation to my device?
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On 14-Mar-17 6:01 PM, James Wilkinson Sword wrote:

More correctly, for sizes under 3", it was originally wrought iron pipe (not tube) and for 3" and above, cast iron pipe. Steel pipe came later.

It should be 0.825" OD.

It is really quite simple. For water flow, it is the bore that is important, so Imperial pipes are designated by that. You then need to add the thickness of the walls to find the diameter of the thread that can be cut onto that pipe. In the days of Imperial measure, things were not done to suit the lowest common denominator.

If you measured a BSP thread that was approximately 1/2" OD, then that is 1/4" BSP. You only need to look up a BSP thread table to find this out.
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Probably, I measured it with callipers and it was just over 3/4". I wasn't being exact, just annoyed it wasn't anything like the 1/2" I was expecting.
I was told they're standard, yet I can see three different sizes on the radiators in my house alone, with most of the radiators having a different size on each end!

So the pipe wall was so thick that the inner diameter was 0.5" and the outer 0.825"? I could understand that with HEP pipe, but not metal.

I cannot find a 1/4" self bleeding valve (only 1/8 and 1/2), but I did find a 1/4 BSP to 1/2 BSP adapter, so I'll try that when it arrives.
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On 14/03/17 18:33, James Wilkinson Sword wrote:

Iron pipe is pretty thick walled.
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To allow for rust?
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On Tuesday, 14 March 2017 20:37:59 UTC, James Wilkinson Sword wrote:

But if you have a filled baguette you can do a good job of tongueing out the filling :-)
Owain
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On Tuesday, 14 March 2017 20:37:59 UTC, James Wilkinson Sword wrote:

No, to allow for the threads to be cut into it. There are three steel pipe wall thicknesses commonly available for different pressures.
http://www.engineeringtoolbox.com/bs1387-steel-tubes-dimensions-d_97.html
The outside for all gauges is constant. The bore varies depending on the pressure the pipe is rated for. This is so all pipes take the same BSP thread when they are threaded.
Hence the bore size quoted is "nominal".
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Make the whole length of pipe thicker just to add a thread on the end?
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James Wilkinson Sword wrote:

Or the middle, or where ever you cut it.
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So we didn't use to have compression fittings, solder, etc? Creating a thread on something sounds like a lot of hard work. And you've wasted huge amounts of material making the pipe thicker all the way along.
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snip

Perhaps you need to measure the internal diameter of your female thread a bit more carefully than "about 1/2" because it might well be 1/8" BSP. The manual one on my boiler is, FWIW. Though the automatic one on the same boiler is 1/4" (or possibly 3/8", I haven't removed it) BSP so perhaps that is what you want.
BTW, your adapter is M to F increasing, is it? M to F reducing is commoner and won't help you!
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The calipers said 14mm, reading the outer diameter of the thread of what used to be in it.

Yes, I selected the female and male sizes seperately from drop down boxes.
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snip

Should all work then! Copious PTFE tape should work for the taper one, the other may need tape, fibre washer *and* sealant if both threads are parallel.
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I know about PTFE tape, but there is something else I can do if the male is 14mm and the female is 15mm?
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hemp and putty used to the used.
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Is I used to using it in a used state? Or what is you thinking?
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On 15/03/17 13:03, James Wilkinson Sword wrote:

I made my son do a PTFE tape joing on a pressure washer part the other day :) Little wibbler will thank me one day!
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On Wednesday, 15 March 2017 13:03:46 UTC, James Wilkinson Sword wrote:

Pipe threads are taper cut, Sealant on the thread is squeezed out as the pipe is screed into the fitting.
Larger pipes are cut with adjustable dies so whatever thread size deemed appropiate can be cut.
Traditionally "Boss White" and hemp was used with water/steam. And"Stag" with petrol/oil.
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