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Just thing about this, I don't do it. So it isn't cool. That's all you should need to know.
Point and case, I had a small gap one time by accident, and when it occurred I got mad chapped lips. Coincidence? (probably) No! Don't do the gap.
I hate to be all mature and grown-up, but if you have that much of a gap it means you aren't wearing your helmet correctly and it will not protect you like intended. But it is probably better than not wearing one.
From what I've been told it's worse. The back edge of the helmet may line up right on a veter bra and hogan leg drop you into a wheel chair, also if it's loose enough you may be just lining your skull up to slap on basically a hard piece of plastic that's supposed protect you.