UPDATE: I made this video before I knew that a smaller GEET works better even in cars. This size may be good for a diesel engines only because more of their total intake needs to come through the GEET or GEETs.
Also although the steel or 400 series stainless is good for magnetic shielding to get the thing starting to work, possibly more electrically conductive metal is better. If the tube metal alloy could be ferromagnetic and highly electrically conductive it might be even better.
If using threaded pipes don't put teflon on the threads because you need a good electrical connection between inner and outer tubes.
This is a video about the importance of smooth mild steel tubes and about having the correct magnetic orientations of the parts. For more information go to:
Regarding the part of the video about magnetic field orientations, some people don't realize a compass' North needle points to another compass' or another magnetized metal's South end.
So they mark the parts, as the North end as pointed to by the North needle & so assembly opposite the diagram, but still with the magnetic field swapping, from rod to inner pipe to outer pipe.
I'm not sure but I think it still may work as long as there is the swapping from the rod to inner pipe to outer pipe.
Its best if nothing is magnetized & initial operation does all the magnetizing.