Smooth copper bushing for 8mm rod. The outer diameter of the bush is 11mm.

Here is a link to this kind of bushing :
http://fr.aliexpress.com/item/Free-shipping-3d-printer-bushing-4pcs-8mm-11mm-30mm-8-mm-copper-bush-graphite-sleeve-for/2009608067.html

Update 14-08-2017: Added a taller version (37mm long)

RepRap

Prusa I3

In order to have enought strenght, do not print flat ; print it verticaly.

Slide the bush in.
Mount it !