Check the height and width by putting them on top of each other, then side by side. When servicing machines, I use metal bobbins whenever I can. However, some machines will work better with the plastic bobbins. If they're the same size, sew a zig-zag stitch with the plastic, then with the metal and see if there's a difference. Plastic bobbins do get rough edges, and need to be thrown away and replaced.