Lisa A. wrote:
"My new-to-me 403a tension dial with numbers just spins. The nut does not
have any pin to fit in the holes on the dial. Is there a pin on the nut
on your machine, to hold the numbered dial in place? I wonder if mine
has a broken or wrong part."
No Lisa your nut is not broken. It does not have a pin in it.
The tension numbers are adjustable. Yours spins, perhaps because you are missing other parts? Does the nut have a set screw like this? Does it have the chrome collar?
In order to "set" the numbered dial you do need to loosen the set screw and remove the chrome collar. Loosen the nut just until the numbered dial spins easily. Turn the numbered dial to the desired setting ( I like to use 4 for the perfect stitch when I am "stitching in" a machine) and while carefully holding the dial at your selected setting, tighten the nut so that it is secure against the numbered dial. Then replace the chrome collar and tighten the set screw.