10 Large Capacity Aluminum M Bobbins for Consew / Juki Industrial Machines Set of 10 High-Quality Large Capacity (M Size) Aluminum Bobbins for Industrial Sewing Machines They offer less resistance which translates to less drag to the bobbin thread tension. This "M style" bobbins measure 1" in diameter and 3/8" thick. Aluminum bobbins are considered the Top-End "Luxury Bobbins"