Mold lapping

Remove the core pin assembly and sprue plate, and block off the sprue hole in the top of the mold. Lightly clamp the mold in a vise, base up. Suspend a 1/4" or 5/16" tap into the center of the mold, and fill with molten lead. When the lead solidifies, you should have a polishing mandrel that fits the inner diameter of the mold fairly closely. Don't worry if there are wrinkles, the mandrel doesn't have to be perfect. Coat the mandrel with valve lapping compound or Clover compound, or something similar, and using the embedded tap, give it a couple turns.