I picked up a spare sewing machine as a crash test dummy for modifications to Mary’s Kenmore Model 158. It’s in reasonably good condition, although the bobbin case showed a bit of rust:
Taking the tension spring off revealed more rust:
An overnight soak in Evapo-Rust got rid of the corrosion and left the pits behind:
Those imperfections on the tension spring are pits, not bumps, despite their appearance.
It doesn’t seem so bad from the outside:
It probably won’t work nearly as well as it should, this being one place where a smooth surface counts for a lot. Fortunately, it’s just a crash test dummy machine and good results aren’t critical.