This is the wet end of the Emerson pump in my Jacuzzi spa. Pictured above is a broken freeze plug adapter. The freeze plug adapter threads into the pump (lower center of pic). It broke with part of the threaded end still in the pump. What is the best way to remove the part that remains in the pump? None of these parts are metal.
There's a tool for just this sort of thing: a sprinkler riser extractor. The pointed tip is pressed into the broken-off threaded part and turned counter-clockwise. The splines on the tool dig into the walls of the plastic part and cause it to rotate. Thanks goes to Lowe's and to Orbit for the photo.