On Friday, February 24, 2017 at 11:28:01 PM UTC-6, yky wrote:
Vanity sink drain usually has two openings for overflow water to go into
the drain pipe. What prevents the water from going down (leaking)
through the thread?

I just installed a new drain and it's leaking. I figure it is the water
going down the thread. Maybe I should use a drain that doesn't have the
opening but the sink does have an overflow hole.

put plumber's putty under the flange lip. otherwise, the water seeps under the lip and down the thread and pops out on the rubber washer. the washer is there to maintain pressure pulling on the flange