The lid is a nightmare my fermzilla would not seal.even found a little larger seal 2.5mm.no joy .I did solve it with this drastic solution .buy a king keg 4"5mm o ring that seals the lid.lube it up screw it down firmly with the tool and bingo .

The lid is a night mare to seal I tried many times even use bigger o ring .still it leaked .I solved it by would you believe a o ring from a king keg 4" o ring takes a bit of an effort to stretch it on the cap.lube it and screw the lid down with the tool.now seals ok