Pipes from 400mm to 800 mm are made from Medium gauge pipe used for water and gas industrial installations.
Pipes from 1000mm and larger are made from Heavy gauge pipe which is used in Industrial Steam installations. This heavy gauge pipe is more expensive and more difficult to find but is stronger and will not bend when clamping larger projects.
Pipes can be threaded at one end, or both ends.
Please see my other listing for sash clamps themselves, pipe joiners and end caps.