I had to remove a threaded plastic nipple that had broken off inside an elbow where it came out of the shower wall. To get the nipple out, I heated the jaws of a pair of needlenose pliers with a propane torch and poked them into the broken nipple (see drawing). Then I slowly opened the jaws, melting slots in the plastic for the pliers to bear against. When the pliers cooled down and the plastic solidified, I spun the pliers with the jaws open to remove the broken part.
Sandy Tod, Campbellville, Ontario, ca