In article
,
harry writes

Pipe sizes are peculiar. 22mm is the nominal bore size. But the bore
size depends on the thickness of the wall. All 22mm pipe are the same
OD but different bores depending on the pressure rating of the pipe.

What a confusing and contradictory statement.

Metric sized copper and MDPE are not in any way peculiar, their quoted
size is the external diameter of the pipe ie. the size of the fitting or
olive required to mate with them.

Andrew has kindly pointed to required olive.

On many plastic pipes, if you are using a compression fitting, you
need a stiffer plastic/metal insert goes inside the pipe to stop it
"collapsing". ie so you get the proper "pinch" effect as you tighten
the nut.

That much is true.
--
fred
it's a ba-na-na . . . .