Special size for Long-Arm Machines – M Bobbin Cases. Easy to use! Just drop a Mega Genie Magic Bobbin Washer into your bobbin case, replace the bobbin, and you’re good to go! Eliminates the backlash and “birds’ nests” on the underside of the quilt when changing directions at high speed. SAVES TIME, with less stopping and starting! Pkg. of 12. Tough and long-lasting.