A buddy had a M6 and before they added the window drop mechanism, it was hard for them to seal because there was no window frame around the glass. On my 840, when you grab the door handle the window drops a bit. I think it also helps when closing the door, it lets the air out so you don't have to slam it. The big coupes have large doors and windows and getting the rubber and glass to line up was a chore.

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