I was planning to cement the loose magnet in my AXN floater Motor and hit upon a a beautiful article:

http://www.torcman.de/motoren/manuals/manual_sm_service_2007_01_ge.pdf (Chapter 3)

The author suggest filling the gap between the magnets with a paste of Epoxy and 'Microballoons'
http://en.wikipedia.org/wiki/Glass_microsphere

is there a common name of these microballoons ? has any body used them ?

TOWER HOBBIES   

Micro baloons are small glass spheres used to reduce the epoxy
density(and quantity used). In your case it does not matter as the amount used
is very small.(Micro baloons also supposedly do not reduce the intregity of the bond).

You can use chopped glass fiber to get more or less the same effect.

And yes ... I have used them !!

indegenize and innovate....

Will filtered fine sand not work?  Seriously, am not joking!

Fine sand - weight!! Micro balloons fill volume and don't contribute to weight. You get cushions and soft toys filled with very tiny balls that are like miniature (really small actually) thermocole balls (like the ones used to fill bean bags. These are really light and may be used. Micro balloons are even smaller though.