Cornelius Keg Fermenter Lid via Adventures in Homebrewing. It has a 3/4″ Hole that’s designed to accommodate a #3 drilled stopper (included) and airlock (also included). That allows you use a Cornelius Keg as a fermenter. Use a 5 gallon keg as a secondary for a 5 gallon batch, use a 5 gallon keg as a primary fermenter for smaller batches or use a 10 gallon Cornelius Keg as a fermenter for 5 gallon+ batches.
I have two 10 gallon kegs that I use as primaries. This allows you to do quite a few things including, easily transfer under pressure, ferment under pressure, carbonate in the primary fermenter and carbonate and serve from the primary (with a cut off dip tube).
As of this posting, this is on sale for $12.99. Check AIH product page for current price and availability
Also: New [unmodified] Lid – via AIH
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