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
Proflowdynamics camlock replacement o-ring size 211 or 210 (see here or here for example) Common supplier: oringsandmore.com, size 210 or 211
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
