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Rob Roy drain cocks

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Rob Roy drain cocks

Home › Forums › Beginners questions › Rob Roy drain cocks

Viewing 8 posts - 1 through 8 (of 8 total)
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  • 30 December 2016 at 12:26 #274418
    Mike Brett
    Participant
      @mikebrett89695

      Hi all

      My 3.5 inch gauge Rob Roy came with four threaded holes in the ends of the cylinders, I assume to enable the fitting of drain cocks. I am trying to find what size the thread is but there is no mention of this either on the plans or in the book. I tried various screws in the holes and the only one to come close to fitting was a 3.4 mm outside diameter screw that went in part way then jammed. As its for a pipe thread I wondered if its possible a taper thread. Any ideas as I would like to fit drain cocks rather than just bung the holes up.

      Cheers Mike

      30 December 2016 at 12:26 #8465
      Mike Brett
      Participant
        @mikebrett89695

        What size thread

        30 December 2016 at 12:46 #274424
        julian atkins
        Participant
          @julianatkins58923

          Hi Mike,

          They will almost certainly be ME 3/16" x 40 TPI, and this is a standard size for commercial cylinder drain cocks on miniature locos.

          Cheers,

          Julian

          30 December 2016 at 12:52 #274428
          Chris Gunn
          Participant
            @chrisgunn36534

            Mike if the original builder planned to use commercial drain cocks these would normally be threaded with an ME thread, such as 5/32" x 40 or 32 tpi.

            Chris Gunn

            30 December 2016 at 13:17 #274435
            JasonB
            Moderator
              @jasonb

              As Chris says likely to be an ME thread which is not often found on screws.

              As the tapping size for 3/16×40 is 3.8mm I can't see Julians size being right as your 3.4mm OD screw would have just slid in, more like chris's. Try poking the shank of a drill into the hole as this will give an idea of the tapping drill used.

              30 December 2016 at 15:25 #274468
              Chris Gunn
              Participant
                @chrisgunn36534

                Mike, I did not think about trying a tapping drill in the hole as Jason suggests, you could try a drill in one of the holes you did not put a screw in, tapping drill for 5/32 x 40 tpi is 1/8", and for 3/16" x 40 tpi it is 5/52". I would not imagine it would be any bigger from what you have told us so far. I can make you a screw to test if it is an ME thread once the New Years festivities are over.

                Chris Gunn

                30 December 2016 at 19:11 #274527
                Mike Brett
                Participant
                  @mikebrett89695

                  Well I tried several drill shanks in the holes and a 1/8 drill seems a nice snug fit. So it looks like the thread is M E 5/32 x 40 tpi. What I will do now is purchase a die for this size and make a small union fitting, if that fits I am home and dry.

                  Many thanks for help. Mike

                  31 December 2016 at 01:11 #274562
                  duncan webster 1
                  Participant
                    @duncanwebster1

                    Mike I've sent you a PM

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