For a high pressure airline, I would agree that this combination is no good at all as the Parallel BSP fitting needs a parallel female thread to screw down onto the O-Ring.
However, you say it's for a Vac line, so the differential will always be less then 1 Bar and you may get a satisfacory seal if the mouth of the NPT thread is large enough to allow the fitting to enter a couple of turns, then PTFE tape may seal at this low pressure. This worked for me recently as a temporary measure until I could get the correct fitting, but the lack of thread engagement was too fragile for me to leave in use. The difference in the thread angles was accomodated by the tape at this pressure.
Alternatively, If I couldn't get a matching fitting, and if I could remove the NPT one, or prevent swarf getting in, then I would consider running a Parallel BSP Tap into the NPT fitting, this would correct the thread difference and allow the O-Ring to do its job as the manufacturer intended.
I wouldn't advise doing any of this if it's a "mission critical" or medical application; but it does seem some pneumatic equipment suppliers expect this type of bodging to occur because many don't supply matching components, even within their own product range!
Trevor Clarke. (R & D) Scientific Instruments.Somerset. UK
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