I haven't opened up many walking feet, but that is where the moving parts are. I once purchased a Penguin walking foot made for the Singer Featherweight at a garage sale. It did not move correctly, and the problem was that the spring on the inside had detached. Once it was put back in place, it worked. The problem I have had with other walking feet is that the plastic shank eventually breaks. Given the choice, a metal shank is a better buy.