I have a JG G36C. I read this on Airsoftparts.ca.:
These are made specifically for the old version Jing Gong gearboxes, which use thinner and slightly wider 6mm bushings. New version JG gearboxes take regular 6mm bushings.
... and it got me thinking; How do I tell which version (They should call it revision so they do not get confused with TM's gearbox versions) of the JG gearbox do I have.
The bushings are plastic. Here are a couple pics- I broke a wire and had to take it apart to solder it back: