Aluminum bobbins are considered the Top-End "Luxury Bobbins". They offer less resistance which translates to less drag to the bobbin thread tension. The metric measurements on these bobbins are 21mm in diameter and 8.5mm deep. These bobbins have a notch in the center for the bobbin winder.  fits Babylock Sewing Machine Models: BL3000, BL4000, BL5000, BL5020, BL7000, BLQP  Bernette Sewing Machine Models: 320, 330, 340, 430, 440  Bernina Sewing Machine Models: 1000, 1000+, 1001  Brother Sewing Machine Models: PQ1300, PQ1500, PQ1500SL, PQ1500SLPRW, XL2005, XL4001, XL5001, XL622, XL702, XL703, XL8015  Consew Sewing Machine Models: 103, 105, 199R, 210, 220, 2230, 230, 231, 292, 99   Elna Sewing Machine Models: 100, 1000, 101, 102, 103, 104, Sew Fun, Sew Green, Sew Zebra  Husqvarna Viking Sewing Machine Models: 2840, Mega Quilter 1200  Janome Sewing Machine Models: 1600P, 1600P-DB, 1600P-DBX, MB-4  Juki Sewing Machine Models: DDL-227, DDL-5530, DDL-555, DDL-5550, TL-2000Qi, TL-2010Q, TL-2200QVP Mini, TL-72, TL-82, TL-98E, TL-98P, TL-98Q, TL-98QE, TR-7  Pfaff Sewing Machine Models: 1200 Grand Quilter   Singer Sewing Machine Models: 103K7, 188K1, 20U, 20U11, 20U13, 20U31, 20U33, 20U43, 206K, 251, 281, 291U3, 306K, 306K25, 306W, 319   Riccar Sewing Machine Models: 2600, 2800, 2900, 291U1, 2950, 3300, 3400, 3500, 3600, 4001, 4003, 414, 420, 500, 515, 525, 550, 555FA, 606, 651, 666FA, 800R, 805, 806, 807, 808E, 8400, 8500, 888, 906E, 9150, 9160, 9600, 9700, 9800, 9900, R551, R552, R650, R651, R750, R751, RE571, RE570, RE580, RE581, RE591, RE640, RM540, RM570