I'm not sure I ever serviced this machine when I got it because there's some tape residue on the vertical pillar that I would've cleaned off if I had ever cleaned and oiled The machine.
As I was sewing on my project, I noticed suddenly I was getting loops on the underside of the fabric. It had me stumped for a minute until I saw that my upper tension dial was at zero. The tension knob was so loose that just the slightest bump moved it from its desired setting.
As I was getting ready to correct that situation, it occurred to me that maybe some others would like to know how that's done so I got the camera out and took some pictures of the process.
There are many styles of tension assemblies, this is the one used on the Singer 400 series. Once you get the external knob off, you find that all upper tension assemblies are very similar inside.
Here is a clip from the service manual telling how to disassemble the 401/403 tension assembly. To merely tighten the knob, you only need to perform these two steps.
After the thumb nut L5 is removed and the split stud is exposed, insert the blade of a flat screwdriver in between the two halves of the split stud Q2 and spread the two halves slightly apart. That will tighten up the thumb nut L5 when you screw it back on.
If you get the stud halves spread too far apart and the thumb nut L5 will not go back on, squeeze the stud halves back together a bit with pliers and try again until you get it where you want it.
I had previously noticed that the +|- indicator was skewed off to one side so while I had the tension assembly open, I thought I would fix that also.
There is a set screw inside the nose door that holds the entire upper tension assembly in the machine. Loosen that set screw and you can turn the assembly to any position you desire. When you have it where you want it, tighten that setscrew and you're ready to reassemble.
Assembly is the reverse of disassembly.
After the tension assembly is reassembled, you want to adjust the tension so that you have a good balanced stitch with no regard to the numbered dial.
Once you have a good stitch, push in the numbered disc D5 and rotate so that the number 4 appears at the top of the dial at the +|- indicator. Release the numbered disc and make sure the pin C5 is seated in one of the holes in D5 and you're done.