Air Fittings Sizing Clarified Explained Clearly

  1. Pollys13

    Pollys13 Member

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    333
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    Wiltshire UK
    I find it quite confusing understanding the different fitting sizes, 1/4 BSP, 1/2 BSP and identifying which size is which. I have Googled but I can't really find anything relevant.

    Perhaps I've missed something I'm looking for clear explanation of sizes and also how to size, measure some fittings I already have, also the American NPT standard. Also a quite comprehensive guide showing various types of nipples, reducers and other air fittings.
    Cheers.
     
  2. WorkshopChris

    WorkshopChris Forum Supporter

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  3. Munkul Member

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    1,192
    Cumbria, UK
    It's a minefield! If you can keep it simple and stick to one thread type - BSP in the UK is the most common, for example - then you just have to worry about connector diameters, hosetail diameters, and hose internal diameters.

    1/4"BSP is the most common thread type on air tools, and most airlines will come in 6mm-8mm internal diameter. 3/8" is a better size to run long lines in, though, and generally takes 9-10mm ID hose. But you can get 10mm hosetails with 1/4"BSP...

    NPT is the same thread pitch, but different enough to cause problems. Often you can bodge it in with thread tape, if needed, but it's not common enough to worry about, unless you buy the very cheapest and nastiest air tools from Asia.
     
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  4. Wany Member

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    46
    Location:
    Gloucester, UK
  5. Cobbler

    Cobbler Codger bodger

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    4,560
    Location:
    Gloucestershire UK
    As a quick check, 1/4 BSP is just over 13mm or 0.51 inches outside diameter, just measured a threaded nipple quickly, but should be close enough to identify it. It is confusing, but 1/4 BSP is for tube with an internal diameter of 1/4 inch
     
  6. bricol Member

    Messages:
    858
    N.Yorks, UK

    Always found Nero to have good info in their paper catalogues - I have one that must be 25 yrs old that is my reference for phyiscal sizes for CAD. But they seem to have it all on-line I've just discovered: https://www.nero.co.uk/Extra/Technical-Information

    You are interested in the nominal bore and pipe size charts.

    All of British Standard Pipe sizes are based on nominal bore of a standard pipe at Schedule 40 - everything else in terms of pipe (not tube) keeps the same outside diameter and changes the wall thickness. This allows the external threads to remain constant - so a thread put on a Sch 10 pipe (thin wall) will be the same as Sch40 (std) and Sch80 (thick wall).

    Like everything, there are more common sizes and therefore fittings are more available in those sizes. And can get reducers or bushes to adapt between sizes. it's all pretty simple really and if you use it a lot, you recognise the sizes as you would say an M6, M8, M10 etc screw.

    There are standards to follow, but in the real world of not wanting to buy any more fittings, I tend to follow:

    Tapered male threads will screw into parallel female and seal.
    Parallel male into tapered female will seal.
    Parallel into parallel should seal on the faces with dowty washers, o-rings on faces etc - but will seal with tape or pipe sealant. Tape or sealant (Loctite 575 for me) helps the tapered fittings.

    Somehow I've never fallen across taper in to taper.

    And as Britain was at the forefront of developing all this, pretty much every other country has standard based on ours. German's DIN, or G (gas), is really BSP. USA NPT is a different angle - but up to about 1/2" will seal without too much hassle -bigger and there's too much interference and it won't screw together very well.

    One mistake I come across regularly is people cutting an external tapered thread too long, or tapping too deep, sometimes preventing other fittings sealing properly as they can't be screwed together far enough.

    One thing I might mention - I'm a mechanical design engineer, but while my colleagues might try to dimension pipe to their usual standards (ie to a fraction of a gnats knackers) I tend to simply use a bigger hammer - or lean harder on a pair of stilsons - so don't anyone get too upset over me screwing NPT into BSP and vice-versa :)
     
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  7. Pollys13

    Pollys13 Member

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    333
    Location:
    Wiltshire UK
  8. Pollys13

    Pollys13 Member

    Messages:
    333
    Location:
    Wiltshire UK
    I have loads of fittings that are the wrong size due to my incorrect assumptions...... me too :) Had a quick look at the link, think that will help identify whats, what.
     
  9. Pollys13

    Pollys13 Member

    Messages:
    333
    Location:
    Wiltshire UK
    I have one of these in its original bag, says Uni hi-flow adaptor -1/4" BSP female
    Same fitting is on Ebay 252449090789 listing description, Hi Flow Euro Fitting Uni Air Line Adaptor Adapter..... 1/4" BSP Female Thread
    I measured the diameter of the female thread and is 1/2 inch, this is where the confusion of the sizing comes in, same with items described as 1/2", 3/8"
    I can't get my head round it, starts to do it in :)
    Thanks to everyone else who replied.
     
  10. Cobbler

    Cobbler Codger bodger

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    The cheapest place I’ve seen hi-flow fittings is Screwfix, they are genuine PCL.
     
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  11. eil Member

    Messages:
    1,753
    N Wales
    When you measure the fitting just subtract 1/4 and you will be near. If you measured the female thread as 1/2" subtract a 1/4 makes it 1/4"BSP.
     
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  12. armalites Member

    Messages:
    4,363
    Herefordshire
    If you are buying higher brands then that isn't the case. If you are buying Chicago Pnuematic or Snap On they will be 1/4NPT not BSP even if you've bought them in the UK. You can get adapters if you want to do things properly or do what most people do and jam a BSP fitting in.
     
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  13. Pollys13

    Pollys13 Member

    Messages:
    333
    Location:
    Wiltshire UK
    OK thanks.
     
  14. Slowcoach Member

    Messages:
    197
    Location:
    Bedford
    If you’re struggling to identify the thread size could you get some normal black iron fittings, 1/4” to 1/2”, maybe nipples and sockets, and use these to work out required size. They often come with the size stamped on them. Probably cost a tenner and save getting the wrong ones.
     
  15. Pollys13

    Pollys13 Member

    Messages:
    333
    Location:
    Wiltshire UK
    Yes I was thinking of getting some from Ebay and keeping them in plastic bags, or use permanent marker so could identify the sizes. Didn't know about the size being stamped on black iron fittings.
    Cheers.
     
  16. earthman Member

    Messages:
    1,163
    It is annoying that there are so many different fittings, all the air tools that come from Lidl for example have the 'Euro' style I believe it's called??
     
  17. Pollys13

    Pollys13 Member

    Messages:
    333
    Location:
    Wiltshire UK
    I think the Euro style, are the shorter version of the long PCL quick connects, think PCL is the correct term.
     
  18. Cobbler

    Cobbler Codger bodger

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    PCL XF are interchangeable with the Euro fittings
     
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  19. Sean Another 602 fan

    Messages:
    1,167
    Edinburgh
    remember when its "Pipe" its the hole in the middle the numbers relate to
     
  20. armalites Member

    Messages:
    4,363
    Herefordshire
    Standard PCL or PCL19 are still the most common fitting but if you are starting a fresh then 100% go PCL XF. Buying genuine PCL fittings is worthwhile. Screwfix sell them at a sensible price.
     
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