On Sun, 26 May 2019 12:23:28 +0000 (UTC),
(Richard Tobin) wrote:

Behind a panel in my house is the exposed end of a long-disused waste
pipe - quite likely there was once a sink there - which I suspect of
being the source of an occasional bad smell.

Presumably I should cap it, but as you can see from this photo it's
not a neat end:

http://www.cogsci.ed.ac.uk/~richard/pipe-end.jpg

The outside diameter is about 7cm.

There are flexible rubber caps for sale, e.g.

https://www.amazon.co.uk/Cloverleaf-...dp/B078HHNFHS/

but I doubt it could be attached securely. Would a piece of some kind
of flexible rubber sheet and a jubilee clip do the trick?

It looks to be lead pipe and the end is quite irregular so no clamp
is going to work well.

The simplest way is to spray it with a small amount of water mist or
wipe it outside and in with a wet cloth and then squirt a bit of
expanding foam

https://www.toolstation.com/soudal-b...ng-foam/p87889

into the end (not too much if the pipe is still attached elsewhere to
used pipes). Put a bit more foam into a plastic disposable drinking
cup and press it over the pipe supporting it in place with some scraps
of wood as it expands and sets. The foam will fill the cup and bond
to the blob in the pipe and the pipe itself giving you a gas tight
seal.