On Wed, 15 Mar 2017 07:42:02 -0000, harry wrote:

On Tuesday, 14 March 2017 18:33:46 UTC, James Wilkinson Sword wrote:
On Tue, 14 Mar 2017 18:24:38 -0000, Nightjar wrote:

On 14-Mar-17 6:01 PM, James Wilkinson Sword wrote:
On Tue, 14 Mar 2017 17:52:00 -0000, newshound
wrote:

On 3/14/2017 5:48 PM, James Wilkinson Sword wrote:
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/Britis...e_thread_sizes

It is pretty straightforward

http://www.bspt.co.uk/

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."

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.

I've bought something stating it's a 1/2" BSP, yet the outer diameter of
its male thread is 3/4", 50% bigger.

It should be 0.825" OD.

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!

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?

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.

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.

Where is this 1/2" measurement taken in relation to my device?

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.

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.

What you want is one of these.
https://www.amazon.co.uk/s/?ie=UTF8&...l_559by5hqwq_b

(Top picture).

Unfortunately the one I bought doesn't reach far enough into the recess! Now attempting to use three adapters in a row....

--
Why is there no Disneyland China?
No one's tall enough to go on the good rides.