On Sunday, February 26, 2017 at 2:16:10 PM UTC-5, dpb wrote:
On 02/26/2017 11:27 AM, trader_4 wrote:
...

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.
...

OK, I can see a misreading of what was I said, but I repeat the threads
on the jamb nut are _not_ the water seal; they're merely mechanical to
apply the force to seat the drainset into the plumbers putty
sufficiently to fill all the voids.

I agree with the above, but that's isn't what you posted:


"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 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. "

You implied that it can't leak below the bowl, except for a problem
with the plumber's putty sealing at the flange where the top of the
drain meets the sink. The rubber seal just above the nut *is a water
seal* and if that seal is bad, it will leak. In his case, the seal
isn't bad, it's that the threads extend too far, so they go past the
seal.




I stand by the previous that if it's leaking there, either there wasn't
sufficient putty used or there's something else preventing it from fully
filling the voids; a non-tapered thread cannot be a water seal and isn't
intended to be the seal in this location.

--

I don't know how I can make it any clearer. There are threads on the
drain pipe, yes? There is a rubber seal between the nut and the bottom
of the sink. There is water present above that rubber seal. If the
drain pipe has threads that extend up the pipe too far, then when the
nut is tight, there will be some threads protruding beyond the rubber
seal, into the area between the inner sink surface and the outer sink
surface, where there is water. Water will follow the threads and leak
out. His drain is meant for a sink that is thicker, where the threads
will not extend out of the rubber seal when it's tightened up.

This is confirmed by his saying that putting silicone on the threads
before assembly solved it. That isn't the best solution, I would have
gotten the correct drain part.