After six decades, the stitch selector knobs were stuck. Frozen by years of old oil that had glued the parts in place. Here's how to fix the parts so you can zig zag again.
How to Unstick the Knobs on a Singer 401 Sewing Machine - Mother
Restoration of a Vintage All Metal Morse R5L “Super Dial” Straight
Vintage: new is not always better!
OldSewinGear News Reviews & How To's - OldSewinGear
The Vintage Singer Sewing Machine Blog: A Singer 401a and Greasy
pungoliving – Professionally Restored Vintage Fine Quality Sewing
What is the best computerized sewing machine? - Quora
Jim's Alabama Sewing Machines
How to Unstick the Knobs on a Singer 401 Sewing Machine - Mother
Jim's Alabama Sewing Machines
Sewing Machine, Mini Sewing Machine, Electric Portable