T i m wrote
Rod Speed wrote

And I can't see how any 'rubber' (that isn't liquid) will ever fully
seal
what is a fairly sharp metal thread, without some additional sealant?

Easy, it just gets compressed into the thread as you tighten the nut etc.

No, it's obvious you aren't an engineer

Much better one than you are, thanks.

or comprehend my point (as you will contradict yourself shortly).

Bull**** I did.

The sharpness is irrelevant if it isnt rotating against the
sharpness as you tighten the nut and that is easy to achieve.

Have you ever actually seen the thread on a metal waste fitting?

Yep. Since I designed and built my own house from scratch on
a bare block of land and did all that stuff myself, personally,
I certainly have done. Didn't use any sealant on any of them
and haven't had even one ever leak at all.

The sharpness (and deepness) of the thread is completely
relevant because you might be trying to 'push' rubber
over the thread profile to obtain a seal.

Nope, you are pressing it into the thread and that works fine.

eg, for the purpose of a good / easy seal, plain pipe would
be the easiest option, then a gently undulating finish and
lastly (and the most difficult to seal) a deep, sharp, thread.

Waffle.

The only exception would be if the rubber had it's own internal
thread that coincided with the thread on the waste fitting

And that is what it has,

So I understand ... and whilst that would work if the exact
same pitch as the metal fitting, may be an issue if the
rubber threads aren't tessellated fully with the metal ones.

But they are when the rubber is compressed into the metal threads.

Doesn't have to be exact, because the rubber is flexible.

A 'spiral' is always more difficult to seal than just rings or smooth.

BULL****. Threads on thread have a lot more surface area
and the rubber threaded section can be compressed into
the metal thread much better than with just rings or smooth.

but even then it would never fully 'sync' (excuse the pun),
depending on how much you compressed the rubber.

That's not right. It's the compressing of the rubber that does that.

Ok, let's say you have a 1 turn per mm thread on the steel and a one
turn per mm thread on the rubber. Now compress the rubber by 50%,
what is the pitch on the thread on the rubber now?

The pitch doesn't change, the compression happens the other
way, compressing the rubber thread into the metal thread.

It is my prediction that it will still leak,

Bet it doesn't.

Time will tell. ;-)

It already has with mine. I never used any sealant and
haven't had a leak in 40+ years. Never had to replace
any of them either.

it might just take a bit of time till it does

Why should it take time to do that ?

Because unless Mike actually tests the seal properly (by applying a
'worst case' positive head of water where the waste is blocked) the
chances are the time it takes to empty the basin into a free flowing
outlet won't 'test' the seal very much.

Time isnt going to change that.

but then may block itself with gunge as time goes on.

Bet it doesn't leak at all.

Time will tell.

It already has with mine. I never used any sealant and
haven't had a leak in 40+ years. Never had to replace
any of them either.

I never sealed any of mine with anything and
none of them have ever leaked in 40+ years.

Like I said, assuming you don't initially expose the seal
to a proper pressure test, over time any potentially
unnoticed seepage will slowly block itself in any case.

Not when the water is relatively clean like in a basin.

At least if you applied sealant to the inside of the rubber bush to
seal against the threads you could still easily remove the nut at a
later date.

And if you don't use any sealant, it will undo fine at a later date.

I have used sealant and never had any issues stripping any
fitting sealed with a non-hardening sealant (even Loctite). [1]

I don't use sealant and all undo fine.

[1] Including safely removing two taps that had been 'set' in putty
and the plumber said he would probably break the basin if *he* tried
to get them out.