Foam Stopper 46-50mm
Breathable foam stoppers will withstand boiling or autoclaving temperatures, making them ideal for use in preparing a yeast starter. The honeycomb structure of the foam stoppers will act is a filter for any airborne bacteria or wild yeast that might enter the flask. That important because a lot of air is sucked into the flask while it is cooling. You can also leave the stoppers on the flask instead of a fermentation lock. Some people use foam stoppers to replace fermentation locks in their carboys, too.
The large foam stoppers are 46-50 mm wide and fit our 2000 ml borosilicate glass flasks.
It is foam and it does stop, so I must give it 5 stars...
Not much else to say, needed it for a flask, worked perfectly.
I found this will fit into most any flask from my 1l upto 3l.
Actual diameter just under 50mm or about 1-15/16".
Works perfectly. Allows the starter to breath and keeps the nasty stuff out