Gymnast2

Though I regularly use a spring dynamometer to set the thread tension of my Singer 201, I had major problems in finding the reason for top tension variations in a range of a factor 2. But finally I discovered the problem related to the design of the -|+ cup of the tensioner. Then I made this video about it. I think the same cup design is used on the Singer Featherweight and the 301. It may be by a case to case basis how much problem this design may cause problems for you. But the fix is easy:
Link to youtube video about tensioner fix

I could not have solved the problem without being able to measure thread tension. It very easy to do with a cheap spring dynamometer, and I think you already know them from school. I know that wrong thread tension cause problems for a lot of sewing machine users, so I recommend to get one or two of them:
Link to youtube video about spring dynamometers

bkay

Thanks for sharing. I'm having some difficulty with the tension on my 401, maybe that instrument would help.
bkay

OurWorkbench

bkay, have you seen https://www.youtube.com/watch?v=WxtQO2XSB4s ? There are some other videos specifically about 401 tensions (one has other bobbins, besides the 401):
https://www.youtube.com/watch?v=UttHpnQy0xk
https://www.youtube.com/watch?v=bTdSkkC4jtM
https://www.youtube.com/watch?v=MwiI_BXanJ8

Janey - Neat people never make the exciting discoveries I do.
Not affiliated with off-site link(s)

Gymnast2

Yes, I recommend to buy two such cheap spring dynamometers - typically $3 each. They have saved time for me every time I change thread or fabric, because it is faster to set the tensions right faster with less test sewing. Furthermore a lot of issues can cause the thread tension to vary, and then it can take a lot of test sewing to find the reason. So such problems are solved much faster. For some meachines it is difficult to take out the shuttle to make some verification of bobbin thread tension. With a dynamometer you can measure it right away by pulling up the thread from the needle plate.

bkay

Thanks, Janey.
bkay