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I have tried 3 wraps of tape and six. Still leaking. Could the problem be these notches? What are they for? They are to perfect looking to be a flaw, right? Toilet valve

Notches

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    I have never seen "notched" threads before. I imagine that the function of those "notches" would be to clean and repair the female threads in the part it is being joined with. The last few threads are not notched and it could be that the valve must be threaded in far enough to engage those last threads. Try using good quality pipe joint paste instead of Teflon tape. Take the joint apart, clean out all the Teflon tape, apply pipe joint compound to the female as well as the male threads, re-installl the valve. – Jim Stewart Oct 11 '18 at 16:20
  • This is a thermostatic radiator valve, right? @JimStewart's theory about the notched threads serving a cleaning function makes sense in this application given that they're likely being installed into old cast-iron radiators with significant corrosion potential. I'd second the suggestions cleaning, then using pipe joint paste and fastening it further in. The key point of pipe dope and teflon tape is not to fill gaps, but to lubricate the connection so you can turn it tighter. On an NPT connection the threads are tapered so the more you turn this tighter it will get. – Shimon Rura Oct 11 '18 at 16:34
  • Now I'm thinking these notches are to help hold the tape in place to help get it started. @Shimon Rura: This is a german-made toilet valve, so it's likely they have some purpose. I switched to pipe thread compound, and it seemed like I got less friction. I could feel those notches as I tightened it with the tape, so they were increasing the friction somewhat. – Tjalsma Oct 11 '18 at 18:01
  • If you got less resistance in threading in the valve with joint paste, then did it thread further in before resistance was finally achieved? Is it now not leaking? Since this is a German made valve perhaps it was designed for sealing with the hemp fiber and paste sealant system used in Europe? – Jim Stewart Oct 12 '18 at 9:16
  • @Jim, yes the joint sealed with using paste alone. I felt I could tighten it further without excessive torque. Next time, I’ll add hemp! – Tjalsma Oct 12 '18 at 13:57
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Since this is a German made valve perhaps it was designed for sealing with the hemp fiber and paste sealant system used in Europe.

In this video the technique is to cut notches, perhaps to engage the hemp. The pipe in this video is parallel, not tapered. Is the valve threaded tapered or parallel?

EDIT

See https://en.wikipedia.org/wiki/British_Standard_Pipe Two types of threads are distinguished:

Parallel (straight) threads: British Standard Pipe Parallel thread (BSPP; originally also known as British Standard Pipe Fitting thread/BSPF and British Standard Pipe Mechanical thread/BSPM), which have a constant diameter; denoted by the letter G. Taper threads: British Standard Pipe Taper thread (BSPT), whose diameter increases or decreases along the length of the thread; denoted by the letter R.

Your valve has the designation "GD1". Could this be straight threads (non-tapered)? Presumably the hose threads on the output are parallel threads so if the "G" designation does indicate parallel threads, then it may be referring to the hose threads only.

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    The valve is tapered. It’s probably made just for countries using NPT. – Tjalsma Oct 12 '18 at 13:49
  • Edited my answer. – Jim Stewart Oct 12 '18 at 14:10
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    The fitting is definitely 1/2” NPT and made for export. It was part of an entire in-wall toilet tank assembly that was purchased on Amazon for installation in Los Angeles. Amazon will not ship anything to California that does not comply with state regulations. I’ve tried to order a 2.5 gallon per minute shower head and was ‘stopped at the border’ when I tried to pay! – Tjalsma Oct 13 '18 at 14:23

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