I have a single pipe steam heat system with radiators. To use this system I use the Maid-o-Mist brand valves. These valves come in different sizes. The website provides a nice diagram about what should go where:

Vent Diagram

And it seems that the difference in the size is just about the little piece that screws into top of the valve.

Valve diagram

Now here is the part I don't understand. Because this part screws in, does it have to be tight? It never says anywhere on the website and it feels like loosening it is an adjustment. But maybe it just loosens because it was easier to make it that way.

Question >> How tight should the top piece be for normal operation?

Link of interest: Product web page

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    Tightening and loosening on a maid o mist does NOT adjust vent rate. – user45452 Nov 11 '15 at 3:31

The particular model in the diagram should have the orifice hand tight. If the orifice is left loose it can vibrate loose and fall off. There are other models and other brands that are adjustable. By turning the cap you expose more or less of the orifice. The larger the orifice the faster the air is expelled. This allows the steam to enter the radiator at a somewhat controlled rate. This allows you to slow or increase the rate that the radiator warms to balance the heat from room to room.

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