Several manufacturers make threaded connectors in various sizes and designs. Some are externally threaded, with threaded plastic caps, both open and closed. Some have internally threaded glass with plastic bushings to hold tubes or plugs, commonly called Ace threads. Solid plastic bushings can be machined for more options.

All these glass threads, in conjunction with their mating plastic parts, can be used to make a tight connection to a straight tube, seal with an o-ring, hold a silicone/Teflon septum for needle piercing, or be used to provide an interface with other materials. You can see a chart of the sizes of GL threads, a page of options for GL14 threads, and a comparison of the internal and external threads. Click the image below to see more of these plastic fittings. We can also provide 1/4-28 male threads, which is a standard size for connecting to assorted small tubing and valves via plastic and Teflon fittings.

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Below are examples of the GL series external threads and plastic fittings. 

Below and right are examples of internal, or ACE threads with bushings

Below is an example of a 1/4"-28 machined glass thread, which fit standard
 small diameter tubing