The boiler needed to fit in a hull 5.5 ins wide. Length restricted to 100mm. I saw a 16g + 103mm + 300mm copper tube on EBay at £9.99 to good to be true. With free p&p what if its rubbish, probably use it for something. Lucky me it came bran new with the British standard marked steam tube, perfect. Held in my lathe on the outside of chuck with the jaws extended & a wooden plug with centre at other end. Not secure enough to machine so just scribed a 100mm length, the cut off by hand with hacksaw. Retuned to lathe & very gently trimmed smooth. Cut the remainder down the centre heated then flattened gave plenty for the end caps. I use ply wood for formers as I have no suitable metal, Turned over to finish as if not careful edges dig in & make it very difficult to remove.(experience)! With the space restriction & the flue has to be the models funnel. Normally a hot back with a return tube would be made. But I decided to be different & return the fire tube on itself. This is made from 28mm tube & fittings. intended for refrigeration use. This is thicker than domestic plumbing materials, as this can very. 6mm cross tubes added after photo taken. To avoid disrupting the flame form the burner 2 ring brazed to outside of tube cross tubes star after these. Although my calculations show, the design could be capable of operating at 70psi it will be restricted to 30psi. Oscillator engines don't like to much pressure, in my experience.