5 Litre water bottles.

[Klemay83]

Do you need a grommet or will the airlock fit in a drilled hole without a one?

 

[Chippy_Tea]

You can drill a 28 mm hole for a standard bung if you get a step drill (cheap on ebay) i have read a 9 mm hole will allow an airlock to seal without a grommet but i have never tried it.

 

[Notlaw]

I drilled an 8mm hole then smoothed and enlarged it with a countersink reamer. It was a bloody snug fitted. I tested the fit and it was watertight, but not sure about airtight. I didn't want to spray it with gasleak detection spray.

 

[engineer]

I have drilled 5 litre bottle tops and used airlocks without grommets in the past.
I just smeared around the base with petroleum jelly which is enough to give them an airtight seal.

 

[Paydoogs]

The grommets do provide a snug fit, but you'll need to drill a 1/2" or 12mm hole in the plastic lid.