Whatever you do, don't grab the shaft with visegrips and twist it. Best way I have found to free it up is to remove the manifold. Start with PBblaster or Kroil and a small hammer. A torch helps, heat the manifold, not the shaft. Sometimes I've only been able to get the shaft to move in and out, but not rotate, which requires drilling out the rivets that hold the flapper. Also, a lot of times someone has tried turning the counterweight with pliers, and has sheared off the pin through it.
