Remove the existing o-ring carefully. Be sure that you do not scratch the vessel’s o-ring groove surface with any tools. We recommend using a plastic or wood stick to remove existing o-ring. Clean all surfaces thoroughly with solvent. Clean the inside of the vessel seal. The inside surface is where the o-ring actually seals. Carefully install the new o-ring in the groove of the cover. New o-rings tend to be stiff and may need to be slowly worked into position. It is sometimes helpful to heat the o-ring in a pot of hot water before installation. This will help relax the o-ring material long enough for installation. Lubricate the o-ring and seal area of the vessel with o-ring grease. A small amount of grease works best. Also lubricate the threads of the nut with process compatible thread lubricant. Thread the cover into the body of the vessel until you feel the resistance of the o-ring being forced into the seal area. Do not try to force the o-ring all at once. Work the threads back and forth gently until the o-ring has worked into the seal area. Continue to tighten until the threads bottom out.