Hi Goran,
Here is the thread (
http://as8.ru/forum/viewtopic.php?f=84&t=18998 -- it's in russian, but use google translate) on installing 310mm rear discs from armoured A8 with some pictures. The guy didn't want the hassle of RS4 install, so went this path with a couple of others. He had to use custom made bolts though, so it's not 100% OEM install.
I personally think normal A8 rear callipers and pads look a bit 'sick' on such a large 310mm disc since not the full surface of the disc is utilised. I think it was Audi's 'makeshift' solution for an armoured vehicle. I DO LOVE thought the brake pad wear sensors part of the story
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