Our old R56 wiper arm eventually came loose at the nut so we had to replace it (and the motor). It cost a pretty penny, but we decided to get the OEM wiper part rather than aftermarket or secondhand. We're glad we did because it is an exact BMW/Valeo part-number match of the old one. (Also, it turns out the the metal cone on the wiper arm (the bit that grips the spindle) is actually smooth on a new one and doesn't have the imprint from the splines yet, so it was actually better than a secondhand one because it will sit snug and exactly where we want it.)