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  1. #1
    Join Date
    Aug 2008
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    Melbourne
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    Question Pipe thread sizes

    What's the deal with pipe threads?? (Seinfeld voice optional lol)

    I needed an adaptor to get from a standard Nitto fitting to push fit hose, so I measured the OD of the thread, which was roughly 1/2", and therefore assumed that I should order a 1/2" BSP adaptor. A few days later it rocks up and it's huuuuge. Turns out a 1/2" OD is actually 1/4 BSP, 1/4" doesn't even vaguely line up with ANY of the dimensions on the thread chart. What the heck?

    Is pipe thread sizing based on some outdated system that used to make sense, or is it just a thing that "is what it is" like number drill sizes?

  2. #2
    Join Date
    Oct 2015
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    melbourne
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    Default

    Best I know, pipe in general goes like this...

    Once upon a time, 1" pipe had an ID of 1", wall thickness 1/4 and therefore od of 1 1/2.

    Then pipe making got better and 1/8 wall was adequate.
    But to retain compatibility with fittings, the od has to stay constant. So your 1" pipe now has 1 1/2 od and 1 1/4 ID.
    Ditto for other sizes.



    Russ

  3. #3
    Join Date
    May 2011
    Location
    Murray Bridge S Aust.
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    Default

    Hi Elan, pretty sure you'll find that pipe thread sizes are refererences to the NB Nominal Bore of pipe. The problem is that there are somehing like 6 different threads BSP Britsh Standard Pipe, NP National Pipe NPT National Pipe Taper
    https://www.valvesonline.com.au/references/threads/

    https://www.ryco.com.au/technical/th...t-nps-threads/

    https://www.engineeringtoolbox.com/t...rds-d_749.html

    It's a very confusing game that one.
    HTH
    Kryn
    To grow old is mandatory, growing up is optional.

  4. #4
    Join Date
    Feb 2013
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    Laidley, SE Qld
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    Default

    It is what it is. The size that each thread is called is based on the nominal bore NB of the pipe. A bit like how the sizes on Whitworth spanners bear no relationship to the size of the hex they fit. Another of those things to get your head around like insert shapes and sizes, types of steel, why did she starting yelling at me, etc.

    What was originally known as British Standard Pipe for the last one hundred years is used world wide (except for 1 country) and has been tarted up with a ISO name. But the nominal sizes, thread count and 55° thread form are still the same as they always have been.


    https://en.wikipedia.org/wiki/British_Standard_Pipe

  5. #5
    Join Date
    May 2019
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    Picnic Point, Sydney
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    312

    Default

    Of course it's going to be confusing because it has a "B" in the name ... meaning British. I'm yet to see a British measuring system that makes sense. Even their money is a cock up.

    Getting back to your thread problem with confusion over the 1/2" dia. British Standard Pipe (BSP) has a parallel & taper thread but also two different parallel sizes ... a straight machine and one for pressure fitting although I don't know if those two are still available. The Yanks have National Pipe Thread (NPT) in parallel and taper & might also have the machine sizes too.

    No doubt there are others that I missed which brings me to special company threads. One I worked at, Pongrass Brothers along with furniture & boats, made air equipment and sometimes used their own threads which meant you had to buy their fittings too. I remember a lot of companies wouldn't buy from Pongrass for that reason.

    Threads ... now there's a can of worms that's best left un-opened.

  6. #6
    Join Date
    Sep 2011
    Location
    Ballarat
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    Default

    Hi elanjacobs
    the key here is the NB (Nominal Bore). It was called nominal because of the different wall thickness of each size of pipe while the outside diameter of each size of pipe remained the same the same. With pipework it is about the volume that can be supplied so it went on the size of the bore. The different wall thickness was about the pressure it could be supplied at. Years ago we would say 'ammonia' pipe, 'steam' pie etc. to denote the different wall thickness which relates to pressures.
    You may have heard someone here refer to what sounds like "shed 40 pipe" which is short for "schedule 40" pipe, or what we would call "Steam pipe". The different wall thicknesses have a different schedule.
    Basically do a rough measurement of the bore to get the thread size.
    Tubing on the other hand is measured from the outside diameter just to make it less confusing.

    Phil

  7. #7
    Join Date
    Sep 2006
    Location
    Mallacoota,VIC,Australia
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    Default

    Basically when working out BSP thread size on a pipe or a fitting simply measure the OD of the thread and take of a 1/4". For example where an fitting has an OD of 3/4" then it is a 1/2" BSP fitting and where a Pipe thread has an OD of 1-1/4" then it's a 1" BSP thread. I'm pretty sure that the same will apply to NPT threads as well. The measurement of the OD's may not necessarily be exact either like you might get an OD measurement of 18.5mm on a fitting but the fitting is still a 1/2" BSP fitting.
    All The Best steran50 Stewart

    The shortest way to do many things is to do only one thing at once.

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