Originally Posted by ajdelange
Yes, but if he has a clean, healthy ferment going the only thing that should be going through that leak is outbound CO2. Even at completion there should be a blanket of CO2 covering the beer and that should stay in place for quite a while as it will take a long time for air to diffuse through the threads.
Yes, but I'm not sure it's a "long time", or rather, I'm not sure how long a time that would last. I have more experience with wine, and a wide headspace is an issue when fermentation slows down and ends. But how long it could be before I have to worry about this, I can't say. I know it would depend on the amount of cracks/openings in the fermenter, the environment (like temperature), and so on. But I have seen mold take hold in a fermenter with lots of headspace in less than three weeks, and that was without any opening to the outside air.