I bought a counterweight a couple of months ago to do exactly that! I had to bore it out about 4 or 5 mm so just I put it on the lathe and ran a boring bar down the middle.
After several attempts, I came to the conclusion that it was case hardened....
I ended up having to throw it out and buy a piece of brass bar and bored that out. Eventually, I actually made up a extension for the counterweight bar instead which works a treat...should have done it in the first place.
