On Wed, 22 Aug 2012 11:11:25 +0100, Andrew May
wrote:

I have just replaced a bib tap outside the house. I notice that both the
one that I took off and the new one have a small screw on the underside
of the tap near to where it connects to the supply.

Removing this screw reveals a small O ring around the screw and hole to
the water inlet.

But what is it for?

Perhaps to drain the tap/pipe for cold spells.

--
Frank Erskine