I don't know much about bicycles beyond basic riding. I am building a bike up from a 1997 Gary Fischer Marlin frame as a learning project.

I just removed the cranks (so I can paint the frame) and it seems like I should clean the area where they attatched. Its dirty and gunky and it seems to be slowing down the rotation of the cranks. Is there a way to remove that small rotating part of the frame to clean it, or is that a bad idea? I'm including a picture to clarify the part of the bike I'm talking about.