Headspace vial, screw top, rounded bottom (vial only) volume These vials require a special cap with an 8 mm thread. The standard 8-400 threaded caps will not fit this vial. The part numbers for caps that are available for these vials are SU8600 (with PTFE/silicone, .3 mm thick), SU86002 (with PTFE/butyl, .6 mm thick),