A couple of extra points:

Suppliers

Finding suppliers for tubes that will definitely fit is proving harder than I expected. In particular I’d like a 70mm diameter by 3mm wall thickness tube for the main tube, as this should fit a 2 1/2″ inner tube with everything at its nominal size. However 70mm is very close to 2 3/4″ so 70mm is hard to find.

I’m also struggling to find suppliers who will measure what they have in stock.

Adjusting the size

I found a discussion on using Car Exhaust Tailpipe Expanders to adjust the size of an aluminium tube. Neat idea but rather scary – I’m not sure what that would do to an aluminium extrusion with a T6 temper.

However:

• It might be possible to make the internal diameter 0.1mm larger, should that be necessary;
• I could always wrap abrasive paper around the tool and use it to make the internal diameter larger using an electric drill on low speed.