Reading the post you put a link to and looking at your problem I surmise that when the screw was fitted in the factory the rad was new and so was the plastic. Taking that screw out which is a self tapper then replacing it, if you don't get it to screw into the same threads it originally cut when it was first fit it will cut new threads in to old, heat stressed plastic which has now probably split as a result. you may find, as suggested, that drying the hole wrapping in PTFE or using sealant may work. if not then new rads are relatively low cost and fairly easy to fit. personally i would do the temp repair until new rad is delivered then fit myself, but that's me, all depends how confident you are..
found this video instruction on youtube on changing radiator
