Preparing the outer & inner core copper tubes to fit together.
I started off with two hours of fun, freehand filing some copper tube to reduce the diameter of the smaller tube as I want them friction fitted together.
The larger 21.96mm OD x 20.40mm ID copper tube is a cutting of plumbers pipe.
The smaller 21.22mm OD x 18.98mm ID tube, a comrade sent to me last year.
This tube needs to be reduced in diameter to fit inside the other.
Tube measurements prior to work:
Outer tube - Outer Diameter - 21.96mm
Outer tube - Inner Diameter - 20.40mm
Inner tube - Outer Diameter - 21.22mm
I filed evenly around the inner core tube using a hand file to reduce the diameter by about 0.84mm.
Pro Tip: Do not use left hand as filing guide 
Inner tube filed down, sanded, light polish and a comparison to an unworked tube.
Tube measurements after work:
Outer tube - Inner Diameter - 20.40mm
Inner tube - Outer Diameter - 20.38mm
Inner tube - Inner Diameter - 18.98mm
Now we have two copper tubes fitting nicely together. 
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