As far as the tension not turning... At some point you will want to pull it apart, clean and re-assemble then adjust. Yes, Tammi has good information on her blog/videos. The 401 and the 500 tensions are totally different and I'm thinking there might be two kinds of 401 tensions. One easier to work on than the other. SSSOOOO be sure to arm yourself with a real good drawing of how it is suppose to go back together for that tension. Then explode it 100% so you can see it well. I used to figure take it apart and put it back together the way it came apart but there are people who go around taking them apart and not putting them back together right so get a manual picture - their mistakes show up at my house... If there aren't two kinds of Singer 401 tensions somebody put one on a machine or two and it arrived here like that... LOL Just getting a little part in backwards or out of order will make it not work right.