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I was trying to fix a slow draining bathroom sink today, so I followed the instructions in this YouTube video. Everything went fine (and it's draining perfectly now), but I can't seem to re-attach the drain pipe (which I believe is called a p-pipe). The part that goes into the wall just doesn't seem to line up properly no matter what I do (and I must have tried 15 times by now). So it just ends up leaking out of that spot immediately.

I have included some pictures to try and show the problem - as I start to turn the slip-joint on that end it just seems to get out of alignment somehow, as if it's refusing to following the correct thread. It also feels like the pipes just aren't really in the correct spot for things to line up, but I guess that's just me not knowing what's going on.

Is there any trick I can use to getting this to line up properly and connect properly? All the videos I've watches just emphasize that you should tighten them by hand, it should be easy, and you shouldn't use any tape or caulking or anything.

Closeup

Entire assembly Another closeup of thread Unscrewed

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    Both pieces must be straight/level with each other. If one is at an angle, you will mess/damage the threads. Loosen all the pieces, so you can adjust the fittings, thread each on just loosely at first till they are all connected, then tighten.
    – crip659
    Jul 16, 2023 at 19:28
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    Also, the vertical connection just to the right of the U bottom looks like one that is adjustable length, so definitely tighten that one last.
    – Armand
    Jul 16, 2023 at 19:49
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    @Armand Extremely good point. The important thing is to not damage any of those (plastic) threads by tightening a cross-threaded (metal) nut onto it. Jul 17, 2023 at 9:57
  • I believe I am seeing a leak in the plastic pipe in the last photo above: a horizontal split (darker than the pipe) between the second and third threads from the top, about two-fifths of the diameter from the left side. That is caused by the nut pulling upwards unevenly, and now more tightening just opens the split further. Best to start over, take the whole thing with you to match dimensions. Jul 17, 2023 at 14:44
  • Are the thread counts even same for the metal software as opposed to the PVC hardware? Anyway, this was messed up before you even opened it, not your fault, but it's your problem! Next time, try connecting the sewer side first since that flanged fitting is annoying and the threaded end just wobbles all over it. It's very hard to manipulate it under constraint to ensure you don't cross thread the compression nut. The verticle section of the p usually overlaps with the down of the drain so close that nut last.
    – AdamO
    Jul 17, 2023 at 15:37

2 Answers 2

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Time to cut your losses...

As little as a new P-trap assembly costs, it might just be worth going to pick one up and replacing everything here.

As noted elsewhere, the metal nuts will chew up the plastic if they're not installed exactly correctly. Once that happens, the threads on the plastic pipe are toast and nothing will get them to be water tight.

Once you've got the new one, follow kreemoweet's instructions about installing all the joints loosely before tightening anything. I'd go so far as to suggest that once everything is connected, tighten each nut 1/2 - 1 full turn and work your way around to each nut before going back to the first one. Just to make sure everything has a chance to settle in nicely before one tight nut prevents the next joint from moving just that last little smidge.

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  • You just need to screw up once, and there's no way to tell you made a mistake until it leaks, but at that point, it's toast and you can't fix it.
    – Nelson
    Jul 17, 2023 at 8:15
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    I figured it's worth a shot (the P-trap replacement was $4.94 from Home Depot) and it totally worked! Thanks for the help! Jul 18, 2023 at 1:13
  • It's definitely incredibly tricky to do this well (even if you do buy a new one)!! It's just one of those things you have to have done a lot, I guess. I can't do it. I always just give up and pay our reliable plumber to come around and do it!
    – Fattie
    Jul 19, 2023 at 11:30
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    Usually, I'll slowly turn the nut backwards, waiting to feel the drop where the thread start points pass, then I'll turn the other way to start tightening. That helps ensure that the threads line up properly and don't cross-thread.
    – FreeMan
    Jul 19, 2023 at 12:45
  • @FreeMan That's also how you make sure metal screws are following the grooves on plastic threads. Why would this happen? Because computer peripheral manufacturers want to screw you over and have you destroy your own stuff.
    – Nelson
    Jul 25, 2023 at 2:33
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First off, it's a bad idea to use metal slip-joint nuts on plastic drain parts which most certainly didn't come with metal nuts. Slip-joint nuts are not all the same, and are not necessarily interchangeable.

One trick to deal with a difficult joint is to leave ALL other nuts loose, so the drain parts have some wiggle, and then try to tighten the problem joint first.

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  • That's a good suggestion. I actually tried only attaching the problematic joint (and not even connecting the others, even though it won't work) and I still had the same problem. So I'm suspecting the thread is messed up somehow, even though it doesn't seem obvious when I look at it close up... (And yeah, the metal slip-joints were not my idea, they came with the house) Jul 16, 2023 at 23:53
  • Although since those are the nuts as supplied I'm not sure that's a helpful suggestion. However at this point I think we need to consider that the nuts are likely to be very weak "pot metal" and might actually be more delicate than the threads on the pipes. Jul 17, 2023 at 10:01
  • @MarkMorganLloyd But who supplied those nuts with that pipe? Jul 17, 2023 at 12:24
  • @RockPaperLz-MaskitorCasket Whoever made the pipes. In any event, they're as OP found them. Jul 17, 2023 at 12:27
  • never even seen such metal nuts. could be "some idiot" previous owner of the house or such put those on there. it's an all-plastic situation
    – Fattie
    Jul 19, 2023 at 11:31

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