Without some form of gasket, Mason jar plastic lids are not airtight. Quality Reliable Products Mason Jar Silicone Seals fit snug into any brand Mason jar plastic lids.  We tried the narrower red rubber seals with the smaller outer diameter.  They tend to become displaced while tightening the lid, and unfortunately, they did not keep our fermentation jars airtight - see photo.  Our reliable seals provide a perfect airtight, anaerobic enclosure while fermenting Kimchi (vegetables) or Kombucha (tea).

QRP SEALS are made wide enough that they will seat against the roof of the cap completely underneath the grooves of the cap and cannot fall out while putting the lid on the jar.  OVERTIGHTENING COULD DAMAGE THE SEAL AND MAKE IT DIFFICULT TO REMOVE THE CAP! 

Metal vs. Plastic Lids use for fermentation and storage of fermented foods?  We find that whatever we are fermenting does not normally come in contact with the lid during fermentation, but however, may briefly come in contact with the lid during storage.  We've done the research, which reveals that metal shouldn't be used during fermentation, nor for storage of fermented foods.