I've been told to use metal bobbins in my vintage machines that have metal bobbin cases and the plastic in newer machines with plastic cases. Basically use what was supposed to go in there when they were made.
I do only use the metal bobbins in my vintage machines, but am seeing more and more vintage machines sold with plastic bobbins in them. I'm just wondering why? I've been told using the plastic in metal cases can affect tension and cause problems. With both plastic and metal still so readily available, why would someone put plastic bobbins in these old machines?