No lights, no radiosity... just HDRI reflecions. Still, it took 7 hours! Maybe because the gears are 512×512 heighfields, and the texture uses micronormals... the render used 343 focal blur samples to smooth them out, and then it was resized down at 50% to help a bit more with the grainy look. I tried first Uberpov blurred reflections, but guessing from the first block rendered, it would have needed a few weeks...
BTW, I reused here the placements from my coins scene using the Bullet Physics Playground... just adapted the gears sizes to the coins sizes.
Credits: scene idea plagiarized from Josef Bsharah image on this tutorial