On Wed, 24 Feb 2016 10:56:38 +1100, "Rod Speed"
wrote:

T i m wrote
Rod Speed wrote
T i m wrote

If the rubber had a thread that matched the waste outlet
at rest, then you compressed it 50% of it's original size,
what pitch would the thread be then.

The compression doesn't happen that way, it sees the rubber
compressed into the metal thread and that is what seals it.

But I'm told there is a thread inside the rubber?

Because that gets more rubber into the metal thread
because the threads match.

R i g h t .... but, if you compress something rubber that's threaded
.... what happens to the thread pitch ... ? Now, it's quite possible
that any thread on the rubber could be designed to match the thread on
the waste when the rubber is compressed to some typical value but if
you have ever tried 'pushing' (rather than screwing) a thread into
place, especially when the other half of the joint is metal and sharp,
the 'threads' may not actually drop into place (potentially making the
seal worse).

I wonder how we ever coped?

By using the much cruder approach of a sealant of some form or other.

Quite ... but it worked

Doing it the other way works much better.

I'm not sure there is a better on 'working fine'. ;-)

No, poor Mike is working under the delusion that 'because it didn't
leak with water trickling though the fittings', then it must be ok
(and I'm not saying it wasn't ok, I'm saying he has no way of being
sure it is).

However, the real (and real world) test would be to see how the whole
lot coped UNDER PRESSURE, albeit only the pressure attained by the
head of water you would see if the downstream waste pipe became
blocked and the basin was filled to overflowing.

I have tested many system like that and (personally) would only dare
to suggest it was 'tested and sound' *when* it passed such a test.

Anything else would be like believing all your gas plumbing was ok
'because you didn't smell gas' versus doing a proper leak-down
PRESSURE test.

Cheers, T i m