These vials require a special cap with an 18 mm thread. The standard 18-400 threaded caps will not fit this vial. The part numbers for caps that are available for these vials are SU860101(with PTFE/silicone, 1.3 mm thick), SU860102 (with PTFE/butyl, 1.6 mm thick), and SU860103 (with PTFE/silicone, 1.5 mm thick). These caps are made from stainless steel and are magnetic.