Internally threaded connectors, available in glass or 18-8 Type 303 free machining stainless steel. For use in place of ground glass joints on flasks, columns, etc. Threads offer positive pressure O-Ring seal when used with fittings like 5837, 5838, 5840-5846, and 7506. Size mm refers to I.D. Of tubing below thread. Fittings for all sizes must be ordered separately.