Here is a great post about that:
http://arcadecontrols.com/BBBB/wc.html
My experience so far is DON’T use a metal Allen wrench.
Out of the three times I have attempted this method I have destroyed the core inside two of them.
One out of three success is not good so I have purchased the proper tools.
I bought a tool set of 4 different sizes here for about $30 shipped: