Its the hole that is 1" in diameter. I bought a fitting at my LHBS that was made for this purpose with the original intention of adding it to a 6.5g bucket, but later decided to give it a try on the cooler. The threaded part of it is not long enough (perhaps only 1.25" long) to go through the cooler and allow the gasket and nut to thread. I spent some time poking around Home Depot but couldn't assemble anything that would work. I was hoping that some online retailer had a $25 device that would fit this, but I can't find one.