First off, all credit should go to Sidd, who's guide I have linked
I recently bought a g502 to replace my g700. Quickly realised the mouse wheel sucked especially when in free spin mode, it is unbalanced so when you stop it rolling it would roll back and scroll up until the heaviest part rested at the bottom.
So decided to search google to see if anyone had any solutions, some one had replaced the g502 wheel with a g500. So I opened up my g700, and it is exactly the same setup as the g502 except the actual wheel in the g700 is probably the best scroll wheel ive used(g502 is cheap shit).
Here is the link to the original guide:
http://blog.siddv.net/2014/09/logitech-g502-proteus-core-teardown.html
This was the only negative in linus' review of the mouse so I thought I would share.
and a pic of mine with the g700 wheel already installed in the g502: