How can I lower a sink trap to accommodate an appliance drain connection?

Question

As shown in the picture below, I'd like to extend a U sink trap to receive drainage from an appliance. However if I replace the dip tube (marked as 1) with variant that has the extra input, the U trap is shifted down too much and the second part becomes too short.

Unless I add some extension piece at (2), the dip tube (1) cannot be longer (cannot be pulled out more) than shown on the left picture. However the appliance drain input of the new dip tube as shown on the right requires me to push the whole U trap downwards, and then I have a gap at number 2.

Is there any piece that can bridge this gap? The U trap has (male) screw threads on it on both ends, and the other pieces are tucked in as dip tubes and are fastened with those female rings. I can not find any piece that would fit there.

One piece I tried looks as below. The female end of this fits the U trap at point 2 perfectly, but its male end is not compatible with the final pipe piece that goes into the wall. The size of the screw thread is compatible but the last pipe piece has a notch that prevents pushing the screw ring close enough to the extension piece that it would fit. (I can add a picture of this if it isn't clear.) It is possible to do it anyway by sawing off a few millimeters of the final piece at connection 2, but cutting it manually doesn't result in a clean enough cut, so it ended up leaking. My suspicion is that this extension piece is not meant to be placed male end upwards and female downwards.

But is there such a piece in existence that is actually designed to fit here?

Answer 1

U traps of this type are meant to be installed unaltered. The problem isn't the trap it's that the waste pipe in the wall was installed too high ( if you have a normal counter top height). The easiest way to fix this is to change the kitchen sink strainer assembly and install one like this. The other possible fix is to find a dishwasher tailpiece that is shorter than the one you have. There are many variations available.

Answer 2

There are two different connections that you're dealing with. The tailpiece from the sink into the p-trap (the connection at #1) is called a slip joint. The connection at #2 is made with a union (and sometimes with a gasket).

I'm not aware of a fitting that would work for you but maybe by looking at this plumbing supplier's website you might find what you are looking for or a least the terms for that you need.