Expanding on what Alexiordan had mentioned earlier, remove the years of oxidation on both contacts of the fan chasis and motherboard by scrapping them with a pointed tool. Dont have to scrape too hard, light scratches on the surfaces will just do. Fold a small piece of Aluminium foil in between the contacts and screw them together as per normal. This would ensure that you will not overtighten, enhance better contact and the Aluminium foil serve as a additional padded conductor. Hav Fun!!