I am trying to figure out what spring you are talking about. I have been sewing all my life and have never had my machines serviced. I take care of them myself. Cleaning and Oiling as needed ...mainly cleaning!!
I have an old single a 15K, that sews like a top. I paid 75.00 for it years ago. I wouldn't trade my singer for any machine.
Maybe you could explain the spring to me.