Try a piece of 1/2" pipe and see if it will fit inside the opening. Many are made so you can solder a pipe to it directly, or use the threaded fitting. If you solder it, remove the cartridge first so the heat doesn't damage it. Same procedure is needed - clean and flux, then heat the fitting (in this case the valve body), not the pipe. Because of the mass, it will take awhile to get it hot - this is where a MAPP gas torch can help, but is not essential, just faster.