Step 2: The frame
Start by cutting the rear triangle from the larger bike, including the seat tube and bottom bracket, Weld a 1.3m section of 50mm x 25mm rectangular section steel tube at right angles to the seat tube 175mm above the bottom bracket. This is now the new top tube for the recumbent.
Next cut the bottom bracket off the smaller frame, this will become the front bottom bracket on the new frame. Weld the bottom bracket to a 1m section of 50mm x 25mm rectangular section steel tube to create the new down tube.
Cut the head tube off the larger frame and grind back the cut sections to leave just the tube. Remove the bearing races and shorten the tube to about 75mm. Drill and file a hole big enough to fit the head tube into the new down tube. This hole should be about 650mm behind the bottom bracket, Weld the head tube into the down tube and then refit the bearing races.
To join the top tube and the down tube first cut a slot 25mm wide in the centre of the front bottom bracket. The top tube fits into this slot and the down tube is welded to the rest of the frame (hopefully this is clear once you have looked at the images). Now file away the top tube that protrudes inside the bottom bracket to allow clearance for the bottom bracket workings.
Next we make the back stay for the seat. This is a piece of rectangular section that is welded to the top tube just in front of the seat tube and to the seat stays on the rear triangle. Fit the wheel to ensure the new back stay will clear it easily. I was lucky enough to find a frame where the seat stays joined to a single tube before meeting the seat tube which made this problem a lot easier.
Finally the frame is completed by adding the head tube. Cut the head tube off the smaller frame and weld it to the top tube. The angle at which it is welded makes it parallel to the other head tube.
To plug the hole in the seat tube I used a rubber foot off a table leg.