Step 3: Drilling out the stem
Remove the bit regularly and clear the swarf, which somehow comes out in copious quantities.
I would recommend working from both ends instead of fighting all the way through. Before each new bit, make sure there is enough clearance that the bit won't go through the side.
I was very careful and still almost cracked the stem as in the second pic. Fortunately that is just a bulge with external splitting, not a crack - it is still airtight. Whew!
Wash the stem out well afterwards with running water. It will be full of vulcanite dust, which you probably don't want to eat.