Originally Posted by Miz Johnny

No--Class 15 bobbins are flat and class 66 bobbins are domed (not doomed, domed). You can find either class in any sewing shop and most fabric shops. I have been sorting bobbins and needles tonight, and I swear to goodness they're breeding.

Also, 66 feed dogs don't drop--at least not easily. Look for a darning plate cover for your 66. And pick up a $5 or $10 Japanese SS machine with dropping feed dogs. It will drop right in your singer base.