Protect your fermentation from oxygen and contaminants with the Twin Bubble Airlock & Bung Set. This essential hardware package includes a high-clarity S-shaped twin-bubble airlock and a #6.5 drilled rubber stopper, specifically sized to fit most standard 3, 5, 6, and 6.5-gallon glass and PET carboys. The "S-shape" design is highly effective at preventing "suck-back," making it the ideal choice for secondary fermentation and long-term aging where small pressure changes could otherwise pull sanitizer into your beer or wine.