Help me understand thread sizes...

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I bought these hose barbs so that I could build a one-man vacuum brake bleeder. They are listed as 1/4" PT male thread, but when I measure across the threaded section, the width is 12.6mm, which is 1/2".

So what am I missing here?

And do I need to use specific PT thread nuts, or will any 1/4" nut do?
 

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If they are American, they will be NPT. This is close to, but sufficiently far away from to be successful 1/4" BSP. What are you screwing them into? If you just need nuts, a visit to a specialist fastener stockist may be needed. Otherwise, 1/4" bsp hose connectors are available from plumbers merchants, with all the attendant fittings. Look at www.bes.co.uk
 
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Screw threads are a bit of a mine field, especially for pipes.
For example the British Standard Pipe thread, refers to the bore size of the pipe and not the size of the thread.
In addition to the diameter of a thread, the form of the thread needs to be considered along with the pitch
of threads
The form and pitch equally apply to bolts, but the diameter is the thread diameter.
So you can't use any nut. A matching nut needs to be same form, pitch and diameter
 
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