Universal NPT/BSP thread - how to cut?
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  1. #1
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    Default Universal NPT/BSP thread - how to cut?

    Hi,

    So as the title suggests, I would like to cut a "universal thread" for 1/4 NPT/BSP fittings. I did not know this existed until I recently discovered some official fittings stating that they are "universal". The first time I came across something similar was on a Chinese hydraulic ram port. When measured the thread pitch gauge for 18 and 19 TPI both rock slightly so not a perfect fit.. Can't remember what fitting I ended up using but recall that it fitted fine. I just assumed it is bad Chinese tolerance but now it seems it might be a "universal thread/pitch".

    Here is an example of the thread: https://www.industrialspec.com/shop/...hread_spec.PDF

    I think the other reference was from a George + Fischer catalog, they claimed it was a patented thread form even though I can't seem to find any patent.

    Edit: Might have been John Guest:STRAIGHT ADAPTOR - SUPERTHREAD | John Guest
    and here (page 3 of the pdf): http://www.cameco-tubings.be/sites/d...0-Guide-En.pdf

    So the question is what is the official pitch/ thread form for this "universal thread/pitch"? I have both 18 and 19 TPI 1/4 pipe thread taps and the thought was to just use both in the same hole, probably not the most elegant way but worth a shot.

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    I believe the term "universal" refers to the fact it will fit all the threads listed in the ad. Not a particular thread spec.

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    Yeah, you will not find it in any standard. That's sort of the point. It's a bastard thread that kinda/sorta fits, but it's probably out of spec (or at best right on the limit) for all standards.

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    Quote Originally Posted by sfriedberg View Post
    Yeah, you will not find it in any standard. That's sort of the point. It's a bastard thread that kinda/sorta fits, but it's probably out of spec (or at best right on the limit) for all standards.
    This. Out of specs and the threaded part is quite short to keep the pitch error in limits. 18.5 TPI and dimensions to loosely fit both NPT and BSP is pretty good guess.
    Its not suitable for anything stressed but low pressure pneumatic fittings are ok. Wouldn't use on hydraulic fittings.

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    These were my thoughts exactly but I don't have a 18.5 TPI tap... Yes, will never use for hydraulics, although the Chinese seem to think it is fine! Just thought it would be handy for pneumatic stuff where not all the fittings are the same when you try and build something. Will try the two tap method and see how it goes,other wise I'll have to shrink or stretch my existing taps to get to 18.5 TPI

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    Wrong thread and wrong thread form?

    Yeah, that does not sound like a winner to me
    Last edited by gustafson; 09-11-2019 at 08:58 AM.

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    Well, FWIW, these guys rate their NPT/BSPT fittings as good up to 1500PSI...

    QPM Products coolant nozzles

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    Could it be 1.4mm??

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    Quote Originally Posted by crystalltiice View Post
    These were my thoughts exactly but I don't have a 18.5 TPI tap... Yes, will never use for hydraulics, although the Chinese seem to think it is fine! Just thought it would be handy for pneumatic stuff where not all the fittings are the same when you try and build something. Will try the two tap method and see how it goes,other wise I'll have to shrink or stretch my existing taps to get to 18.5 TPI
    SMC Pneumatics (and Parker-Legris IIRC) also make these "Universal" pipe threads to fit NPT, BSPP & BSPT. I expect they are 18.8 or 27.5 threads per inch, they are tapered, and they are short (~4 threads) so they should fit either pitch female port. Seal is achieve via a 45° metal washer, vs. on the tapered thread itself. Itf you were making them (and couldn't turn the non-standard 18.5 tpi, I'd guess, for the short length of engagement, either standard pitch tapered thread would work (pick the standard w/ the slightly larger diameter, as they aren't quite the same?). at least at reasonable pressures (< 100 psi?) I've never used them at pressures above about 15 psi though.

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    In reality, people force npt fittings into bsp holes and vice versa all the time without knowing better. Worst case is usually a fitting that weeps a bit, and a hole that needs it's threads chased at some point in the future.

    Assuming these are just a half pitch off and made of a soft enough material that it will conform to the mating part, it will probably take the same pressure as a correct fitting without too much trouble.

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    Thread sealing compound!

    As for cutting 18.5 TPI, with manual change gears I can cut 18.46 TPI (26 and 60 teeth) or 18.66 TPI (24 and 56 teeth) on my SB. One of those would probably work, perhaps both. Exactly 18.5 TPI would require a custom made gear or a custom compound gear in the train. There are ways to calculate that: I leave it as an exercise for the reader.



    Quote Originally Posted by gregormarwick View Post
    In reality, people force npt fittings into bsp holes and vice versa all the time without knowing better. Worst case is usually a fitting that weeps a bit, and a hole that needs it's threads chased at some point in the future.

    Assuming these are just a half pitch off and made of a soft enough material that it will conform to the mating part, it will probably take the same pressure as a correct fitting without too much trouble.


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