Durable Class 66 metal bobbins with curved sides and four holes for classic top-drop-in systems. Robust construction supports stable tension and long service life across a wide range of fabrics. Suitable for some Brother, Baby Lock and Singer machines, but not for Pfaff, Janome, Elna, Bernina or Husqvarna models. Supplied in a 125-piece bulk pack for studios, service departments and high-volume sewists who need consistent performance.