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What's the correct way of using flexible tap connectors with normal UK domestic plumbing?

They're sold at most plumbers as a convenient way of connecting bath taps, ball-valves and showers etc. The normally come with one end that looks like this.... flexi-coupling

Which can be screwed straight onto a normal 1/2 inch or 3/4 inch connector on the underside of the tap. Like this..... tap

Sometimes this works fine, but often the threaded brass connector on the tap doesn't really seem to be designed to fit the flexi-connector. The end (of the tap) is often quite sharp and jagged, and unless I spend five minutes beforehand filing and sanding it smooth then the rough metal on the tap cuts into the rubber seal in the flexi coupling causing the joint to leak.

The taps seem to have been perfectly designed for connection to 15 or 22mm copper pipe using an olive, a nut and a length of PTFE tape but they don't appear well suited for flexi-connectors. Am I really supposed to file them smooth by hand before connecting to them? or am I missing something? Is there an extra washer I'm supposed to fit instead?

Any help appreciated.

Update following comments - I have measured the width of the flattened end of the tap tails of an old bath-tap. It's hard to be accurate but it's about 0.3mm. This is pretty narrow, it doesn't seems to take much over-tightening before it starts to cut into the rubber. My technique is normally to tighten finger-tight, then turn the mains water supply back on, then repeatedly tighten quarter-of-a-turn till the dripping stops. Then after a while i realise I've cut into the rubber of the washer.

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  • We use the same compression fitting in US plumbing, but the compression fitting is on the wall fitting side, not the fixture. Could this also be the case for UK?
    – blarg
    Jan 8, 2023 at 13:18
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    That end is designed to just tighten on like a shower hose, 1/2" or 3/4" BSP. The other end is a proper compression joint to fix to standard 15mm copper pipe. I've not seen tap tails that weren't smooth enough to just do this, like the ones in your 2nd pic, but I don't do this for a living so my sample set is small.
    – Tetsujin
    Jan 8, 2023 at 14:37

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The flexi coupling should thread easily on any fixture of the same size. Do NOT use any type of tape product (PTFE or similar) This can only be more of a problem. The flexi is made to be used without any other sealer, but if you must use a small amount of Teflon paste to lubricate the threads.

My experience with these leaking is caused by either under or over tightening. If you are cutting the rubber seal you are over tightening. These are not made to be tightened like a compression coupling. They should be tightened by hand until firm. Then another 1/4 to 1/2 turn only.

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