Filament Polyester . Fil-Tec Bobbincentral's patented magnetic core creates consistent delivery through the entire bobbin. No more interruptions for tension adjustments as your bobbin gets smaller. Magnetic core prevents backlash or over-spin and eliminates the need for backlash springs. Magna-Glide is the original high-yield magnetic-core bobbins. These Class L bobbins are suitable for most domestic sewing machines.
