Yes, you do need flat, not tapered, washers under the bolt head.
Just for you I've been out and taken a bolt off my C5.
The washer is held in place by some ridges rolled into the top parallel section. This top section is about 17.5mm diameter and about 8mm from under the washer to the start of the taper. The taper is about 4.5mm long and goes down to 12mm diameter.. This is threaded for 17.5mm of its 23.5mm length.
Total length from under the washer (which is 3mm thick) to the end of the thread is 37mm.
C5 ALLOY WHEELS AND DASH DISPLAY
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