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I have to draw two custom pipes, 1 1/4" & 2", with on one end male tapered pipe thread and on the other end parallel pipe thread. The connections are Parallel female thread with a tapered male thread, with Teflon tape as sealant.

I'm familiar with the metric tread, but the British/ Whitworth thread system is new for me. After a search on the internet and the standards: DIN 3858 / DIN ISO 228, i have the following questions:

  • According to DIN 3858, tapered thread largest available size is 1 1/2". Why are there couplings on the market that are larger? In my case 2".
    • Is the designation of "Pipe thread DIN 3858 - R 2" correct?
  • What is the minimal thickness of the pipes when it concerns tapered thread? Are there guidelines for this?
    • For the 2" connection i selected a OD = 60.3 mm, t = 2 mm. (probably too thin)
    • For the 1 1/4" connection i selected a OD 42.4 mm, t = 2.6 mm.
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After some discussion with an experienced college and a continued search, I'am able to answer my own question.

DIN 3858 is valid for type C stud ends as in DIN 3852-2 and type Z tapped holes with parallel internal thread as in DIN 3852-2. Example Type C stud -click-, page 19.

In my case i'm looking for pipe threads and not thread on a stud. For this case EN 10226-1:2004 is valid. Also a references was available in DIN 3858. This guidance show the full pipe thread range until 6". This explains directly the bigger available sizes.

The wall thickness questions can be answered with some common sense. The wall thickness must be larger than the complete thread heigth incl the height created by the taper. Say roughly 2 time thread thickness. From the steel pipe catalog, i even found some process pipes specifically for 2" & 1 1/4". 60.3 x 3.65 & 48.3 x 3.25. These fit with properly with the thread and my selection was wrong in conclusion. These were thin wall pipe suitable for compression couplings.

It took me some time, but some good lessons are learned for future work.

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