"Richard" wrote in message
...
Friend is removing one rad and replacing it with two others. In lifting
the floor he has found that the main supply around the first floor is in
thin wall steel pipe with what appears to be 15 mm copper tee-ed off to
the actual rads. I've measured an offcut of the steel as being 21.5 mm OD
with walls of about 2.5 mm.

WTF????

SNIP
Hi Richard.
At 21.5 you will find it is 1/2" Nominal Bore PIPE (NOT TUBE). (actual size
0.840" OD or 21.336mm).
You could cut a thread on it or look for a 1/2" Nominal Bore compression
fitting.
Baz