The gears Miriam mentions get brittle with age and Murphy's Law dictates they will shatter when no other machines are available and you're in the midst of a deadline project with only a few hours to spare. To avoid plastic (or nylon) gears in Singers, stick with models before the Touch & Sew (600 series). The first T&S models (there are several amusing interpretations of these initials, presumably invented by disgruntled owners), the 600s, had metal gears but you'll have to inspect to be sure.