"Tool-less" serrated clamps add about 75mm to a 20" depth and make it impossible to mount on a flatter rack (although at 21.5" I had plenty of room). Despite the table on the Silverstone website for these rails listing dimensions for 18" rails, they also don't appear to exist. If you don't have a square hole rack, it comes with screw hole adapters, but using these will reduce your depth by an additional 48mm (two on each side). Max is just over 28-1/4", whatever that is otherwise they mount perfectly and move smoothly.
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