38mm Poly Seal Screw Caps (fits most 1/2 & 1 gallon jugs) [Bag of 12] Go to amazon.com Related posts Top 10 best carboy cap Feature12 countFits most 1/2 gallon and 1 gallon jugsThe poly cap will NOT rust over time DescriptionThese 38 mm poly seal screw caps fit our 1/2 and 1 gallon jugs that we carry. The poly cap will not rust over time.