Unfortunately, the thread running to somewhat conical. And when you have a not so well-made mother, it caught just a little. As for me a mother leaving only 1/3 turn it, I had to sand down to about 3/10 mm with some sandpaper the thread of the rod. The mother can now turn it without problems, but just not in the others. The rubber on the nuts should also be removed, they smell a little chemistry and when man has weights on it and the dumbbell moves a little too fast, the weights of course try to turn around and thanks to the rubber is excellent, the force transmitted to the nuts and rotate several revolutions. When one hires the rubber rattle the weights a bit, but no longer rotate.
