15mm 1/2" pipe differences help needed

Discussion in 'Plumbers' Talk' started by ajohn, Jul 13, 2018.

  1. ajohn

    ajohn Member

    I'm having memory problems on this.

    I recollect that 15mm compression fittings can be used on 1/2" pipe but that the olive will probably be tight. Not sure which way round this is. A loose fit sounds like bad news.

    I have an L shaped piece of 1/2" pipe with 1/2" compression fittings on each end that I need to replace. I doubt if the threads on these match modern 15mm fittings but it would be wonderful if they did ??. Could I use 15mm pipe and 15mm olives to replace it in the existing 1/2" fittings. The only other alternative without replacing a lot of pipe would be to block off one end of a 1/2" T with a 15mm stop end - the type that goes into an existing fitting.

  2. candoabitofmoststuff

    candoabitofmoststuff Active Member

    1/2" pipe refers to the internal diameter, 15mm pipe is the external diameter...

    In my experience they are so similar that you can even use solder fittings without a worry, so using 15mm olives on 12" pipe isn't a problem. Occasionally they are very marginally tight, but not enough to be an issue.

    However, the threads don't match, but your solution to block with a stop end will work.

    When it comes to 3/4" and 22mm... they are different enough that solder fittings aren't interchangeable.... but the compression fittings are so long as you use an olive suited to the pipe...

    i.e. you can use a 22mm fitting with 3/4" pipe, so long as you use a 3/4" olive.

    I'm DIY not pro, but that/s my experience anyway.

    Good luck,


    KIAB likes this.
  3. ajohn

    ajohn Member

    Thanks. There is a gate valve on the pipe from the 70's. Probably never been shut off and will probably leak from the gland now if it was. Can't really do much on it's inlet side so best option seems to be to leave it and add a decent ball valve into the new piece of pipe.

    The only other option would be to find some fitting that would fit the 1/2" nut and the olive on it that could then connected to 15mm pipe. It strikes me that the "usual" suppliers don't give much thought to people who have imperial pipe work and may need to replace parts and as there is still lots and lots of it about they should.

  4. ajohn

    ajohn Member

    It looks like a part can be bought to fit into 1/2" nuts/pipe with olives leaving 15mm compression on the other side. Similar available for 3/4 and 1" as well.


    Just hope that the hex on it is as large as shown in the photo to keep it still while tightening up. There is a1/2" end feed joint on a T rather close to the nut that was probably wiped.

  5. ajohn

    ajohn Member

    :( Just bought some of the connectors I linked to in 3/4 as it would make another job easier - straight to 22mm in one go. 3/4 pipe wont fit into them and the thread is smaller than the thread used on the 3/4 coupler it would have been part replacing - just using one of it's nuts plus the existing olive.

    :eek:Also bought a couple of the 1/2" to 15mm I linked to. 15mm pipe goes in with some clearance so I live in hope that 1/2" will too and that the threads will match existing 1/2" nuts. Have to undo one to see. Had enough for today.

  6. Pollowick

    Pollowick Well-Known Member

    Where you are using 1/2 & 15mm select copper olives. They are softer and deform a lot easier.
  7. ajohn

    ajohn Member

    The 1/2" to 15mm nearly works, 1/2" pipe fits, thread on that end is slightly undersize compared with 1/2" bsp and seems to be the right diameter for old 1/2" compression coupler nuts :mad: but the pitch is different so wont fit.

    So looks like my rather old seized up gate valve is here to stay or the entire pipe would have to be replaced.

    Brass - copper olives. I fitted several cheapest parts that use brass olives without any problem on 15mm and the some time ago same on 1/2" but when it all fitted I thought the 1/2" must be 15mm. No problems but I do use a bit of sealant. If I don't do that they make creaking noises while being tightened and needs a lot more force. The lubrication from the sealant stops that and reduces the force needed. I couldn't get sealant neatly on one yesterday and had to do it up dry. Then having found my old ducks foot found that the ends no longer fit compression fittings. Sad that as another spanner can be used on them. People sell them as 15 and 22mm as well. The ones around here are actually 1/2" and 3/4 and do fit old compression fittings.

    3/4 to 22mm olives - I would want copper as brass does harden as it's distorted and there is more to compress.

    OK I am a DIY'r but they work for me. I'd agree that olives have changed but they still seem usable for me. Not to the extent an old local plumber did on our outside tap years ago. Tightened just enough so that when the pip froze the pipe popped out of the olive rather than bursting the pipe. He was still around and told me to pop it back in and to be careful not to over tighten it. He commented that people always do which is why the pipes burst. If alive he might be in is 100's now.


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