Oh if it were so simple as to say that it's NPT, BSP or JIS for that matter. There are more standards than you can shake a stick at for "Tube Fittings". [img]images/graemlins/bs.gif[/img]
Here is a PDF from Eaton (they do lots of automotive components).
Hmmm so what is the common standard for DOT brake line? Is there a standard? [img]images/graemlins/confused.gif[/img]
Actually there are two standards (sorta), one is the old SAE spec'd components (likely what you need) and the other is the metric version (for the most part JIS). [img]images/graemlins/censor.gif[/img] And yes there are parts made to convert between the two.
Just for a different flavor the Aircraft industry has there own standard... likely superior to most of the rest (good idea if you are going to fly) but NOT DOT approved.
If you really want a mess... start with a British designed machine, with lots of pipe/tubing. Then get some half whit folks here in the USA to transfer the manufacture of that same machine to China... Wow that hurts even ten years later... Never mind. [img]images/graemlins/wall.gif[/img]
I toss all of this out because Tube Fitting threads don't comply to the standards of easy to find taps. It is all specialty machining. [img]images/graemlins/mad.gif[/img] [img]images/graemlins/thud.gif[/img]
My best suggestion is that you buy a proportioning valve from eBay if you can't make it seal up. Then again maybe there is a board member that has a spare for cheap.