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the steerer and headset on the A-D are BSC dimension ("English")
however the wall is a bit thicker than a regular BSC steerer yielding a stem diameter of 22.0mm rather than the 22.2mm one might expect, while the metric/french size is an actual 21.9mm
the manufacturer also has a steerer size unique to them of 26.0mm O.D.
afaik this is not employed with any of the models of the A-D range
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