I'm using rubber bungs (with airlock) for my 5l glass carboys, I just got the carboys so it's my first time fermenting in them. I have a problem that rubber bung keep getting out because of the CO2 pressure from the fermentation.

How do I keep the bung inside? Are silicon bungs better for this stuff?

  • 1
    You are using a drilled stopper with an airlock, right?
    – bughunter
    Commented Jan 26, 2015 at 13:01
  • 1
    Yes, edited the post to reflect that.
    – freiksenet
    Commented Jan 26, 2015 at 13:35

11 Answers 11


Duct tape....I've done it more than once.

  • 1
    I don't know why I didn't think of that. Time to go get some duct tape. Thanks Commented Jan 26, 2015 at 17:17
  • This is just ingenious.
    – freiksenet
    Commented Jan 28, 2015 at 7:31
  • 1
    I guess it's the 40 years I spent in the rock'n'roll biz. Duct tape is the answer to everything!
    – Denny Conn
    Commented Jan 28, 2015 at 16:34

I have these bungs: enter image description here

Soft-ish rubber bung on the left, stiff plasticky rubber in the middle and soft silicone on the right. Silicone seems to be the least slippery when wet, but the soft rubber works OK. The one in the middle, OTOH will come out if you look at it funny.

Your bung should require some effort to remove. If so, then make sure you don't some kind of blockage. Or reduce the depth of water so there's less pressure.

  • Indeed. Using just a couple inches of sanitizer in your blow off bottle is key, depending on size of the bottle a full gallon of saniziner can translate to a few psi the bug has to hold back. Commented Jan 6, 2016 at 13:18

Good 'ol fashioned weight works for me ;-) My bung for my blowoff hose kept popping out, so I just stuck this book on it. I know you're talking about an airlock, but just thought I'd share what my fermenter looks like right now. Also, are you doing blowoff at all? I usually do blowoff and then switch to an airlock after the fermentation dies down a bit. I'm thinking that after initial fermentation there may not be excessive pressure on your airlock.

Carboy with book weight

And yes, that's a t-shirt around the carboy to keep the sunlight out.

  • After reading jsled's answer, I would have to agree. I wonder if the bungs would even pop out without any CO2 pressure. Either way, a little weight will probably do the trick. Just make sure there's nothing stopping the CO2 from getting out, or you could end up with a fermenter bomb! Commented Jan 26, 2015 at 15:14
  • 1
    That is a heavy looking blowoff tube. I use a piece of copper tubing to make the bend, then the bung has little/no torque on it.
    – Pepi
    Commented Jan 26, 2015 at 16:01
  • @Pepi thanks, I might look into that. Cause yes it's only the torque that's making it pop out Commented Jan 26, 2015 at 17:15
  • I just use airlock, I leave generous head space (1.5l in this case) for the krausen. I think I got even too generous this time - small batches don't foam all that much.
    – freiksenet
    Commented Jan 28, 2015 at 7:33

Have you tried Carboy Caps? http://www.northernbrewer.com/shop/3-5-6-gall-carboy-cap-orange.html

I've had the same problem and switched to the caps and find they work Great

  • I have 5l (LITER) carboys, I think those would be too big. Also I am in Europe and northern brewer is not available :(
    – freiksenet
    Commented Jan 28, 2015 at 7:32

It's also important to note: 5 gallon carboys usually use #7 bungs and 6 gallons use #6.5. The main risk here is that you'll sink the bung into the carboy if it's too small, but I imagine if it was too big you might have difficulty getting it to stay in.

Also, I find it works best to fully insert the airlock in the bung, then insert the bung in the carboy, but again, this is mostly to prevent sinking the bung, but it will also increase the size of the bung a bit, over only having the airlock partially in.


Wrap the top of the carboy and bung with some aluminum foil. This should add just enough pressure to hold it in place until it dries. Plastic wrap may do the same thing.


Let the sanitized bung dry (preferably away from dust) before inserting. It fits more snuggly that way.

Edit: I've also reused cork cages from Belgian beers with good results. Just extend out the bottom so it fits all the way around the carboy neck.


I'd say that a Silicone bung would be the best thing to go for as Silicone has a lot of chemical, weathering and heat resistance meaning that it will stand up to things like this. Try http://www.vital-parts.co.uk/silicone-tapered-plugs-336-p.asp, this is where I usually get mine and they work perfectly for the application.

Just make sure that when you insert the bung that its dry and fits in firmly without any movement.


This is a common problem. I think a summary recap answer is warranted.

Proper Stopper Size and Style. There are dozens of styles. Hoods, drilled plug, molded silicone etc. Pick one you like, they all work just some easier than others. Most important get the right size for your carboy.

Hold it In. Tape, foil, plastic food wrap, champaign cork retainer wire, palet packaging wrap.

Reduce Pressure Don't over fill the blow off bottle. Couple inches is all you need. The depth of the sanitizer translates into psi the stopper has to hold in.

Clean and Dry Make sure the stopper and carboy neck are dry. Clean paper towels work well. Using a sheet that you unwrap to get to has worked well for me.


How about this... Use your carboy handle.

Tighten the wing nut so the handle is hard to move. Adjust it to hold the Bung down like this:

carboy Handle

8 Ways to use a carboy handle


The stopper does not need to be fully "inside" the neck of the carboy. It simply needs to rest naturally in the neck, and its own weight should hold it in place. Any CO2 should easily be relieved through the hole and through the airlock or blow-off tube; there should not be enough pressure to move the stopper at all.

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