Here is a scan from a book that is no longer in print:
...the picture tells the story.
The coil shown in the photo is meant to go into a chimney pipe for heat recovery, so the precise internal and external dimensions are not so critical. If the coil was intended to wrap around a waste pipe for heat recovery, the internal diameter of the coil would be critical. When wrapping such a coil, a certain amount of "spring-back" will occur, so some experimentation would be required to make a mandrel (a log in the picture) of such a diameter so that contact between the formed coil and the copper waste water pipe would happen. Then, soldering would really improve the situation.
Regards,
-AC_Hacker