Welcome. Even though I do have a Willcox & Gibbs, I'm not sure I can help. I have a bunch of small open end wrenches, but do not remember which size is for the nut. If memory serves correctly, it is a
tapered nut.
After oiling the nut from the top, I would be tempted to put a piece of rubber (like bicycle tire tube) around the nut and use a small pair of pliers or vice grips to try to remove it. Once off, I would (or get someone else to, LOL) file some sides on it, so that a wrench could be used. There are some tools to remove rounded bolts, but don't know if they would go small enough or convenient to use on a sewing machine.
Perhaps a picture would help and maybe some might have some ideas and correct dimensions.
We like pictures. In order to post pictures on Quilting Board, you will probably need to reduce or compress the picture to a
file size that is smaller than 2MB. How to post images can be found at
https://www.quiltingboard.com/attach...020-01-17-.pdf
Don't forget to scroll over to the far right to find and click on the "
Upload" button.
I have found that only the big red "
Reply" button or "Quote" work to give me the paper clip icon to work for adding pictures.
https://www.quiltingboard.com/attach...p-location.jpg
Some additional info regarding reducing images at post #4 and images for the scroll and upload button can be found at #5 of
quilt block ID?
Janey - Neat people never make the exciting discoveries I do.