MotoGP God
Posts: 18,275
Join Date: Apr 2002
Location: Oxford
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Forget the X2 bit, but yes the offset is measured from the mounting face to the outside braking face surface. Most 320mm 6-bolt fitting Ducati discs are 10mm offset with a few as Antony mentioned are 15mm.
As Antony is going to a Radial Caliper fork you can't space the Calipers to suit so you either need the discs with the right offset or space the discs out, which is what I have just done to make a Ducati Narrow Track Mag wheel fit the Radial Calipers on my 998R
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