"Usenet Nutter" wrote in message
...
On Mon, 25 Jan 2010 11:24:54 +0000, Dave Osborne
wrote:

Er, I think you'll find that in a tap connector, the fibre washer stops
water leaking down the thread.

Or UP the thread :-)

I suspect that some people use PTFE tape on threads - when the thread is not
designed to do any sealing. In applications like tap connectors the washer
on the shoulder provides the seal - on a compression fitting - the olive
provides the seal. Clean faces and threads are a good starting point for
getting a seal - also well aligned pipes.