O-Rings are used to seal the different fittings on a cornelius kegs to prevent carbon dioxide from escaping.
O-ring sizes for common Homebrewing applications
Silicone O-rings
Cornie lid o-ring size 417 Common supplier: McMaster-Carr part #9396K926
Cornie post o-ring size 111 Common supplier: McMaster-Carr part #9396K24
Cornie dip tube o-ring size 109 Common supplier: McMaster-Carr part #9396K74
Weldless o-ring for 1/2" bulkheads size 211 Common supplier: McMaster-Carr part #9396K32
Buna-n O-rings
Cornie lid o-ring size 417 Common supplier: McMaster-Carr part #9452K218
Cornie post o-ring size 111 Common supplier: McMaster-Carr part #9452K23
Cornie dip tube o-ring size 109 Common supplier: McMaster-Carr part #9452K172