Stopcocks come in a variety of forms; they all have a borosilicate glass shell, with a plug which effects a seal, and can be opened to some maximum bore diameter. This diameter is used as a reference for size; a 4mm bore through a teflon plug is referenced as a "4mm teflon stopcock"
These are component parts selected for their utility in any given design, then sealed onto the glassware. Scroll down for a few examples of context.
Ground glass stopcocks have a glass plug with a hole or holes through it, allowing closure or opening with a half turn of the plug. Ground glass stopcocks require stopcock grease to lubricate and seal the plug and shell. This can be a problem if there is a danger of contamination from grease. Silicone-based and hydrocarbon greases are available; if we get a vote, we like the latter – they burn off cleanly in our ovens when we need to repair them.
There are stopcocks in the same form as above that feature Teflon plugs. This allows for sealing without grease, and are probably the most common kind. They are not as effective for high vacuum, although they work for low vacuum applications, such as vacuum filtration. They are typically held in place with a washer/oring/nut assembly.
There are a variety of configurations of arms and bores, such as the small sampling presented below...
Threaded stopcocks and valves have a glass shell with a threaded end which accommodates a plug which screws over (or into) the threads, sealing the stopcock into the shell with o-rings, and effecting the seal at the tip of the plug. Teflon plugs are most common; glass plugs are available in some forms. Some plugs have extended conical tips which provide greater control of the aperture for flow.
Shown on the left is a zero dead space bottom valve for a jacketed glass reactor that ports through the teflon stem.
You can see lots of examples of these components in our extensive photo galleries. Or scroll down for some selected illustrations of their context.