On Wed, 24 Feb 2016 06:20:53 +1100, "Rod Speed"
wrote:
snip

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


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?

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. 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.

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. A 'spiral' is always more
difficult to seal than 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?

It is my prediction that it will still leak,

Bet it doesn't.

Time will tell. ;-)

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.

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

Bet it doesn't leak at all.

Time will tell.

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.

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]

Cheers, T i m

[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.