I was removing part of the bobbin "stuff" on a singer 646. I got the black screw out perfectly. (I borrowed my Dh's Chapman screw driver set. ) The (apparent) aluminum screw on the next level would not move. So, thought, well, maybe it's one of those backwards screws. It moved about a 1/4 turn, but then it stuck. I tried it again the other way, but it moved 1/4 turn and stuck again. I had to work today and got home late, so I'm not sure what the Pb blaster did to it. I'll try again tomorrow.
bkay