In message , Dave
writes
My rising main is black alkathene (I think), the outside diameter is
close to 21.5mm - how can I find out what the bore is without removing
the fittings?

Well if you'd asked yesterday....
Just this morning I put a new stop tap onto a the end of an c. 1970
alkathene pipe. As in a previous thread, I'd measured the pipe (with a
vernier) in situ at 22.0, but when I cut the end off this morning, a
22mm olive was a loose fit, so it was probably 21.5mm. Wall thickness
was about 3mm.
I might be able to find the off cut of the scrap pile tomorrow if you
need more accurate than that. I successfully re-used the copper insert
that was in the end of the pipe.

--
steve