Cicero wrote:

Scaffolding tubes are usually 2" diameter.

Not quite - ali scaff tube is actually 1-29/32 in. dia. by 7 swg wall
thickness (48.3 x 4.47 mm in new money). The reasons for this odd size
are doubtless historical - but if anyone out there knows why, it would
be interesting to share the secret.

"2 inch aerial masting" usually is a genuine 2 in. dia by 16 swg wall,
but I have also come across 48.3mm x 16 swg, so caveat emptor.

--
Andy