On Sunday, February 26, 2017 at 11:12:32 AM UTC-5, dpb wrote:
On 02/25/2017 5:31 PM, yky wrote:
...

I think you are right. It's not the correct part. It comes with a new
faucet I bought. I'll just buy a new drain.

After reading your earlier post, I think we were talking two different
issues...I was "underneath the impression" it was the tailpipe
compression nut but I think I now understand you're talking about the
jamb nut under the bowl that holds it mechanically in place.

If so, that is _NOT_ a water seal; the problem if it is leaking there
isn't the nut/washer, it's that you either don't have enough plumbers
putty to make a full seal


It is a water seal. Look at the link to a diagram that I
posted. A typical sink has two drain paths, one that you're
talking about from the bowl that the water usually goes down.
The other is from inside the sink, between it's inner and outer
layers, whereby the overflow is routed. The sink drain has
holes in it so that it's open in that gap inside the sink,
below the hole, so that the overflow water can run into it.

What he has is a situation where the drain pipe has threads
for the nut going too far up the pipe, so that when it's
tight, some portion of the threads at the top are in that
gap area, where there is water, even if the sink isn't
overflowing. Water then follows the threads, gets passed
the rubber seal under the sink and leaks.




or the drain flange isn't seating in the bowl
opening properly or it's got a bend or something wrong with it or
there's an imperfection in the bowl itself.

There should never be any water on the underside of the bowl itself that
has a chance to come out at that location; if there is, the fault is in
the seal between the drainset at the top and the bowl opening, _NOT_ in
the connection at the bottom side which is mechanical ONLY.

--

Not true, per above. I made that mistake initially too, but
when looking at the diagram and thinking about it, I see
exactly what he's talking about.