These pistons measure 1mm lesser diameter on the top compared to the bottom. I have documented this with the last piston from the 535 Continental UCE. Technically I do not know why this is the case but if I had to venture a guess I would say that the top has more thermal expansion than the bottom.
Based on the company’s official 86mm stroke the actual displacement based on the BIGGER DIAMETER (because that ultimately determines the size of the cylinder jacket) are calculated using this online app.
477cc real displacement
432cc real displacement
411cc real displacement
511cc real displacement