Rubber stopper

I’d appreciate suggestions on the following: I have a rubber
stopper for a flask. The hole in the stopper is much too small
for the Baume hydrometer I want to insert through it. How does
one go about enlarging the hole in a rubber stopper?

Thanks in advance,
Judy Bjorkman

In the Chemistry Lab we used to Lubricate glass tubing with
glycerin and slowly twist it into the hole in the rubber stopper.
You might give this a try if the hole isn’t drastically smaller
than the tube being inserted.


hello Judy, Use a wire that is the right size, get it red hot
and push it through the hole you want to enlarge. Have fun. Tom Arnold

Have you tried drilling it out or make a cut down the side of
the hole so the hole can expand as you push the tube. If you cut
along side of the hole, I don’t think you will have a good seal.
Good Luck, Lisa

I haven’t actually tried this but I think you can freeze this
material (Is it really rubber or is it neoprene or what it’s that
hard black stuff that comes with chemical glassware, right?) and
then it is somewhat more likely to respond favorably to a tapered
reamer or a hand turned drill. Just a guess, j

Dear Ms. Bjorkman,

I usually use a drill. It's that simple. But don't try to hold

the stopper in your fingers while you drill - put in is a vise of
something similar first.

	Michael Knight

F.E. Knight, Inc., 120 Constitution Blvd., Franklin, MA 02038 |
508/520-1666 |

The safest way is to find astoper with the right size hole.
Inserting glass tubing in a rubber stoper is dangerous enough
without trying something haphazard. BUT you don’t want to put a
stopper on a hydrometer. The devise is designed to have a given
weight-density adding a stopper will defeat the purpose of the
device and destroy the calibation. The devise must float
unimpeaded to work. Jesse

Chemists use cork borers to make holes in corks. (they’re sort
of like small cylinder cutters, and come in sets with all
different sizes). But I can say from past experience that
enlarging a hole that is too small can be a hopeless task. Think
it would be more practical to see if you can find another
(hole-less) stopper of the right size and then see if you can
find a friend in a chem lab (maybe in a local high school – or
college – that has chemistry classes) to make a hole the right

Hello Judy

It can be very tricky to enlarge a hole in a rubber stopper, so

I suggest you get a new stopper from a lab-supplier. The easiest
will then be to get them make the hole. If they won’t, you can
proceed as follows: Take a piece of brass tubing with o.d. the
size of the wanted hole (or a little bigger) and about 2" longer
than the stopper. File and buff one end very sharp. For easy use
you can solder a piece of brass rod 4" long to the other end.
Then place the stopper on a piece of wood and gently squeese the
brass tube through the rubber, turning it slowly as you squeese.
Lubricate in between with household detergent.

Hope htis will help you, I have made several holes in rubber

stoppers this way, but ended up buing a set of proff. “drills”,
which basically are all the same as the above, just with a little
more finish to it. Goo luck.


G’day; the simplest method for you would be to use a fairly
coarse round file, starting with a file a little smaller than the
original hole. I’ve done a ‘few’ that way.

The ‘correct’ way to bore rubber and cork stoppers is to use a
tube sharpened at the end, lubricated with good old spit or more
hygienically, with glycerol, but NOT any oils. One can buy sets
of what we used to call ‘cork borers’ varying from 1/8" to 1",
and available from lab suppliers. Memories! I must have bored
literally hundreds of corks and rubber stoppers in my time. When
I was a lab boy (in 1936!!) I used to have to get apparatus ready
for whole classes of junior chemistry students every day, and
that included vast amounts of cork boring! I hated it!
Cheers, –

        /\      John Burgess
       / /
      / /
     / /__|\
    (_______)  In sunny temperate Mapua NZ -

Autumn’s here and winter’s close