Sewing machine bobbins, CB shuttle, steelThe bobbin hook is at the heart of each sewing machine, because it is responsible for the knotting of the threads and therefore for the seam. Proven over many decades, the CB hook for the central bobbin is the most widely used system and is ideally suited for home use. Prym provides these CB bobbins in metal or transparent plastic. Both versions have a diameter of 20. 5 mm, a height of 11. 7 mm and can be supplied as plastic bobbins with 4 per unit