I may be wrong, but if your std mount is for a 140mm disc, then a 180mm (+20mm) adapter will allow you to fit a 160 disc, not a 180. Please feel free to correct me on this.
As VFR400 says, 20mm radius.I'm a little more confused now
That means the diameter of the disc is a gain of twice that, 40mm, the difference between the diameter of a 140 and a 180 mm disc.
If you buy the part VFR400 recommended from ebay, you'll be ok.
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