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I tried to connect the needle valve to plastic tubing, but the tube slides out if I pull on it. More over the metal no longer fits over the tube because it has already been tightened. needle valve image

Since one side no longer fits I tightened the other side to stop the water for now but for sure this will not hold. What is the right way to go about this?

  • You have done it the right way in your picture. Compression fittings need to be tightened such that the ferrule actually squeezes down onto the tubing, never again to be removed. I suspect that your initial try was not tight enough (yet enough to distort the ferrule; you will need a new one...) – Jimmy Fix-it Nov 18 '19 at 5:56
  • you could try to slide the ferrule onto the tubing first and insert the brass tube after – jsotola Nov 18 '19 at 6:02
  • The ferrule is inside the bolt, it does not come out. I think it is not meant for metal on plastic scenario, simply because I can just wiggle it loose without much effort at all. On the first attempt I made it quite tight... – user584572 Nov 18 '19 at 14:53
  • Once attached and tightened it also rotates quite freely around the brass tube... Not going to experiment until I get more clarification (running out of fittings :) ) – user584572 Nov 18 '19 at 14:56
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Your assembly looks right. The tubing could be nicked, though, so when you try again snip off an inch from the end of the tubing so you'll have a fresh surface to work with. Also verify the mating face of the valve is clean and undamaged.

One common cause of leaks with poly tubing (at least it used to be for me) with the use of a brass ferrule is that the ferrule can get misaligned a little too much. It gets compressed a little out of round and won't seal. It's more prone to happening with poly tubing because poly is softer than copper tubing. The job definitely can be done with a brass ferrule; I'm only saying it can be a little challenging and don't feel badly that it has taken a few tries.

You could try the connection with a Delrin (aka acetal or "plastic") ferrule as illustrated in this answer. They're a bit easier to work with on poly tubing. These ferrules are generally available alongside the other fittings for tubing in a plumbing department.

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  • Second attempt worked better. Seem something got misaligned on the first try. Plastic ferrules also work well. Thanks! – user584572 Nov 26 '19 at 3:19

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