No, the 66 comes in metal also. You should get metal if the machine comes with a metal bobbin, plastic if it comes with a plastic bobbin. The newer machines are made in such a way that metal bobbins will wear out the casing. I don't know how vintage your machine is. If it's really vintage, then it was designed for metal bobbins. If it's a newer more plastic type Singer, then it may have been made for plastic bobbins. I'm not sure how you tell the difference with a used machine. With a new one, you always know because of the bobbins that come with the machine.