heat the fitting and/or around the casting. then apply a birthday candle, allowing the wax to generously melt against the hot surface[s]. the wax will wick into the joint[s] like solder does when soldering copper piping. you may be pleasantly surprised at the results. you may have to do this a couple of times though. i remove all of the 5-40 slot head screws on the cowl vent screen on my 33 dodge humpback panel truck using this method, even though many of them had badly messed up slots in the screw heads. all but one turned out as if they were installed yesterday and were new.