I’ve purchased a new bathroom sink faucet on Amazon, and I’ve installed the set. But I’m puzzled by a rubber ring near the bottom of the drain body that came with the set. I couldn’t figure out what it’s made for. The second picture shows that I’ve removed it (did not install). Does anybody know what it does?

enter image description here In the picture above, I was pointing at the o-ring near the bottom of the drain tube (not ring A nor B) it’s the same o-ring I’m holding in picture 2 enter image description here

  • 1
    Your question isn't very clear. What you're pointing at in the diagram is typically a thick wedge shaped rubber gasket to seal the drain assembly to the sink, and what you're holding up appears it may be a gasket to seal the drain assembly on the top side to the bowl of the sink so it only drains when you unplug it. If you forgot the topside gasket, you can loosen the drain assembly, push it up and put a ring of plumbers putty around it before you tighten it back down and clean away the putty that squeezed out. If you forgot the bottom side seal you should take it apart and reinstall it.
    – K H
    May 7, 2021 at 3:33
  • 1
    it's neither A nor B. What I was pointing was the rubber o-ring as it’s shown in picture 2
    – jeffrey
    May 7, 2021 at 3:50
  • Right. A and B both end up on the bottom side of the sink. The manner in which the tube connects to the trap and the purpose of the O ring is not shown in the instructions.
    – jay613
    May 7, 2021 at 12:00

3 Answers 3

  • Ring labeled "A":

Multi-function tapered rubber gasket that centers the drain assembly in the bottom sink hole, cushions the clamping action of the nut that secures the drain assembly, and seals the lower hole (most sinks have an overflow drain-hole and channel cast into the body of the sink that leads to the space between the upper main drain-hole and the lower main drain-hole, overflow liquid enters the drain assembly through small holes in the drain assembly within that space).

  • Ring labeled "B":

This is a plastic (sometimes metal or fiber) friction ring; contradicting it's name, it prevents friction between the nut and the rubber gasket above so you can firmly tighten the nut without the nut grabbing and twisting/turning the rubber gasket.

  • P.S.- the rubber gasket (A) is often called a "P.O. Plug gasket" or "pop-up assembly lower gasket". May 7, 2021 at 1:20

It's an o-ring. It's there to act as a seal to stop water leaking from the junction of the metal drain body to the plastic drain pipe. Not fitting it may cause leaks. You can turn your faucets on full and see if you get a leak.

  • Is my installation going to lead to problems in the future? I didn’t have the o-ring in there
    – jeffrey
    May 31, 2021 at 22:49

Perhaps the O-ring provides a seal in some other kind of drain system in another country. I've never seen one like it on a drain tube. If the white compression nut shown has a wedge shaped compression washer under it, that's all you need and the O ring isn't part of the seal. In any event it can't do anything because your drain tube is too short! It barely makes it into the trap. Hopefully if you do have a proper compression washer under the nut, it has enough tube to do its job, otherwise you should buy an extension.

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