Big reason I'm not liking the new Big Mouth Blubbers for aging... too much head space in the big necks. had a couple batches get nasty bacteria on top. Hopping to save them with good 4x meta solution I think it alos has to do with wall thickness of the glass No joke, I use to top mine right up to the bottom of the cork, or within a 1" of it. Not anymore. With heat they expand and will rise up into your airlock.