The chain is only as strong as its weakest link, so too is the beer barrelling process; I have risked the welfare of more pints of homebrew than I care to admit by allowing old ‘O’ rings to blight my quality safety record, moral of this Tip - Keep a supply of new and unused rubbers! and be sure to throw the old ones away! even if they appear in reasonable condition, (you know it makes sense!)
This 4" seal is not to be confused with 4" Wine Fermenter Cap Seals - (different cross section). but either seal, different as they are, will benefit from a light application on all sides with specially formulated, Pressure Barrel Seal Grease available from Balliihoo to ensure a good airtight seal with no fear of Contaminating the brew.