How to get better seal on a 5/8" dishwasher drain line adapter?

Question

I have a kitchen sink drain that has a y-branch for dishwasher drain water. I think the y-branch is 5/8" diameter. This is probably the one I have:

https://www.homedepot.com/p/Oatey-1-1-2-in-x-12-in-22-Gauge-Brass-Hi-Line-Sink-Drain-Tailpiece-Extension-Tube-with-Nominal-Sweat-Branch-HDC813B-3/316622018#overlay

I want to hook up a reverse osmosis drain adapter to it, and the only ones I can find end in a 7/8" diameter (because I think they want to hook into a garbage disposal, which I guess usually have a 7/8" opening).

I've bought two adapters to step down the 7/8" RO end to the y-branch 5/8" end. When I tighten the worm screws, I never get a good enough seal on the 5/8" end, and it leaks.

This is one of the step-down adapters I bought as an example:

https://www.amazon.com/dp/B08172WTYJ?ref=ppx_yo2ov_dt_b_fed_asin_title

Any ideas on how I can get a better seal? I can install a drain saddle but hoping to avoid drilling into the drain pipe to install that.

Thank you

Answer 1

Try a 1 1/2" tailpiece with a true 7/8 branch Like a Poly tail piece.

You may need to install a drain tailpiece and then the tail with adapter into that. It's an extra piece, but that will correct your issue.

Answer 2

The size of the Y branch is the same as nominal 1/2" copper water tube ("pipe"). Solder on a reducing coupling (1/2"x3/4") and you will have a 7/8" OD tip on the Y branch that will fit your hose.