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BSP Threads

Tuesday, 11th May 2021
Graham (Tech Advisor)
The Welders Warehouse Online Shop

BSP Threads are used for most Oxygen, Acetylene and Propane equipment.

Fuel Gases, Acetylene, Propane, Polypropylene etc will have Left Hand Threads because this stops Fuel Gas and Non Fuel Gas equipment from being mixed up, which is potentially VERY dangerous.

Left Hand Threads will have small cuts machined into the Nut to indicate they are Left hand.

Fuel Gas has Left Hand Thread

Non Fuel Gases Oxygen, Argon, Argon/Co2 Mix etc, will have Right Hand Threads and so no cut on the Nut.

BSP Thread Sizes

The size of BSP Threads causes a lot of confusion because, for example, a 3/8″BSP Thread is over 1/2″ in diameter!

BSP stands for “British Standard Pipe” and is a very old thread.

In the example of 3/8″BSP, 3/8″ is the diameter of the BORE of the pipe NOT the outside diameter of the thread.

If the BORE of the pipe is 3/8″ diameter, clearly the outside diameter of the pipe has to be greater and it’s this outside pipe diameter that a thread would be cut into. So the Thread diameter has to be greater than the 3/8″ BORE diameter of the pipe!

BSP Thread Diameters

Gas equipment typically uses 4 different BSP Threads, the following is the basic specifications of these threads.

SizeOutside DiameterPitch
1/8″0.383″ (9.73mm)28 TPI
1/4″0.518″ (13.16mm)19 TPI
3/8″0.656″ (16.66mm)19 TPI
5/8″0.902″ (22.91mm)14 TPI

BSP Thread Uses in Gas Equipment

1/8″BSP is commonly used for the outlet thread on small Regulators, the type used for throwaway cylinders for example.

1/4″BSP is typically used on the Torch end of Oxygen, Acetylene & Propane Hoses where small torches, including Lightweight and Model ‘O’, are used.

3/8″BSP is typically used for Regulator Outlets, Flash Arrestor Inlet + Outlet and the Torch end of Hoses where larger Torches, including Heavy Duty and Cutting Torches, are used.

5/8″BSP is typically used for Gas Cylinder Outlet and therefore Regulator Inlet.

I hope you found this article useful.

Visit our Gas Welding Tools & Fittings page.

Please let me know what you thought of this article by leaving a comment.

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Regards

Graham

20 comments

  1. Alasdair Maciver says:

    very interesting and informaitve thanks

    1. Graham says:

      You’re welcome Alistair, glad you found the article/information useful.
      Regards Graham

  2. Siva says:

    Oxygen gas regulator connecting nipple outer Dia14mm what is the connector size when i am going to purchase to the shop. Kindly help me

    1. Graham says:

      Hi Siva
      I’m sorry, I’m going to need more information to answer that question!
      Use the Contact Us page of our website to contact me directly via email and I will do my best to help.
      Regards
      Graham

  3. Samuel Farnes says:

    I have come across many argon / nitrogen regulators which say they have a 5/8” BSP connection to cylinders. But there are many different types of 5/8 connections, and for argon / nitrogen I believe a BS341 no 3 is required. So how do I know if a regulator has the right connection?

    1. Graham says:

      Hi Samuel
      I am not aware of any different “British Standard Pipe” (BSP) Threads! There are, of course, other types of 5/8″ Thread, but they would not be “British Standard Pipe”. Full spec for 5/8″BSP is:
      OD=0.902″
      Core=0.811″
      Pitch=0.0714″
      Depth=0.0457″
      Radius=0.0098″
      Effec=0.8563
      TPI=14
      ODP=15/16″
      Hope that helps
      Cheers Graham

  4. Ken Johnson says:

    Bought on eBay Argon regulator strugled quit a time found the thread diameter was .960″ 55 thou too large approx . Full refund and told to keep regulator . So is 960 strange continental or what’s going on . ?! Ordered another indicated thread size 22.25 mm
    Hope all ok . Any info interesting

    1. Graham says:

      Hi Ken
      Hazard of buying on ebay I guess!!!
      If it’s a new Regulator your buying, make sure it’s a reputable brand from a reputable supplier before you put it on a cylinder with in excess of 2000 psi of pressure! The few Pounds you’ll save on ebay is not worth the risk of an accident with high pressure compressed gas, or combustible gas.
      If it’s a second hand regulator, be VERY careful, or better still, cancel the purchase.
      To my knowledge, UK BSP Threads are not used anywhere else, so buying gas equipment from outside the UK is always going to carry some compatibility risk.
      Cheers
      Graham

  5. David Rae says:

    Hi. Thanks I always wondered why this was the case. Funnily enough I looked this up after reading an article on why bike chains are always on the right! PS You mention polypropylene as being one of the flam gases. It is a solid. No need to keep this comment, so do delete. Thanks again for the info

    1. Graham says:

      Hi David
      Thanks for the comment. My bad re Polypropylene, should red Propylene didn’t spot the incorrect spell correction!!!! WordPress doesn’t seem to have Propylene in its dictionary. I will go back and try to find all the “corrections” and correct them back to Propylene 🙂
      Now I’m wondering why bike chains are always on the right!!
      Cheers Graham

  6. Rick says:

    In the gases used above you did not mention butane. I have a propane bottle for my gas forge with standard fittings. I wanted a brazing torch which i got on ebay,(eeek). The torch that i bought was being sold for butane and identical looking torch’s were also available for propane. The butane torch thread did not match my propane bottle. still lefthanded but wrong thread. Is that because it is from ebay and imported or because butane uses a different thread size?

    1. Graham says:

      Hi Rick
      I regret I’m not overly familiar with Butane equipment (beyond the disposable bottle equipment). I would expect Torches to be much the same as, apart from flame temperature, Propane and Butane are similar in that their both Liquefied gasses.
      The issue may well be that you’ve bought it on eBay. A LOT of eBay gas equipment is from suppliers who produce for the American market. American Threads are typically UNF type (mostly 9/16″ UNF from what I remember), here in the UK, standard threads on gas equipment are BSP.
      If the issue is with the Nut on the end of your Torch Hose, you may be able to change it. You will need to know the Hose Bore size in mm. Have a look at our Nut & Tail sets.
      Always best to check threads with sellers before buying gas equipment off eBay.
      Hope I’ve helped
      Cheers Graham

  7. donald calvin says:

    hi there,
    So lucky to have found your thread.
    Does everyone use the BSP, as a Standard?
    Or the Americans use a different Thread.
    Are all Oxygen tank/Cylinder have one thread the 5/8″BSP as a standard?
    if so, all gauges/regulators have the same and will be compatible.
    Thanks much,
    Don

    1. Graham says:

      Hi Donald
      BSP threads are UK standard, I’ve yet to come across a UK supplier of Gas or Gas Equipment using anything else.
      Other Countries have they’re own threads and are not compatible.
      The Americans, for example, typically use a 9/16″ Thread for torches, hoses etc.
      I hope that helps.
      Cheers Graham

  8. Simon says:

    Thanks Graham, this cleared up some confusion about whether a new argon regulator was the right size. These BSP size designations are a pain to deal with if ypu’re not used to them!

    1. Graham says:

      Hi Simon, thanks for the comment. BSP threads are a constant source of confusion 🙂 I wrote the article after I had a customer get quite aggressive on the phone. I told him his thread was 3/8″BSP, but he INSISTED it was 5/8″ because he had measured it. It took a while to explain, he then thought it was a stupid way to measure a thread, like it was my fault! 🙂
      Cheers Graham

  9. Adekunle says:

    Thanks graham for the timing info. More Grace

    1. Graham says:

      You’re welcome, glad you found the info useful.
      Cheers Graham

  10. Peter Smith says:

    Excellent info,Graham, helps judge size immediately. Thank you for your informative blogs.

    1. Graham says:

      Hi Peter
      Thank you for your comment, much appreciated, and you’re very welcome, I’m glad you found the information useful.
      Best Regards
      Graham

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