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Both the BR-c3000 and BR-c6000 seem to come with either a 3/8"x7.2mm, 3/8x8.2mm or 3/8"x 9.7mm washer it seems to go between the Nexus8 hub and the brake.
Part 6 on the lower diagram
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Dibble-
Ah, right; the rollerbrake bikes I've got came with them already installed. IIRC the rollerbrake fits directly onto the splines on the hub; the fixing washer then goes on top of that, with the NTW and cap nut tightening down on that fixing washer. Sheldon just says the various washers are for different hubs, but the Shimano techdocs are silent on the matter. I think it depends on the frame spacing and the OLD of the hub you're going to use - it looks like the Nexus hubs have OLDs from 127 to 135mm, as per this chart via Sheldon: https://web.archive.org/web/20130925001938/http://www.shimano.com/publish/content/global_cycle/en/us/index/tech_support/tech_tips.download.-Par50lparsys-0015-downloadFile.html/08%29%20Hub%20Dimensions.pdf
Thrustvector
Which washer do you mean? IME rollerbrakes are ok; not massively powerful, but consistent in all weather conditions, and need pretty much no maintenance from one year to the next.