I've uploaded all of the files both separately and in an assembly. I'm sorry your head didn't work out. The one I printed has a little groove in it that it catches in in truck mode. I don't know why exactly yours doesn't work. You are correct that it has loose joints. This is to ensure nothing fuses together when printing (I went through three prints before I got to this version, with the working joints). However, both truck and robot mode should hold themselves together fairly well despite this.