There are many different bobbin sizes in use today. The most common sizes are the L Class, M Class and Class 15 bobbins. A large percentage of commercial and home sewing/embroidery machines use one or more of these bobbin sizes. Chances are your machine does too, but the difference between them is sometimes confusing.
Of the three classes I mentioned, two are distinctly different and not interchangeable. The L Class and M Class bobbins are substantially different in size. Machines designed to use L Class bobbins will not accept M Class bobbins. The M Class bobbins are much larger in diameter and will not fit into the bobbin case. The graphic below illustrates this point.
As you can see the diameter of the M Class bobbin is 1/4 inch larger than the L Class bobbin. There would be no way to use a M bobbin in any machine not designed to accept this size. It would be physically impossible. On the other hand, the L Class bobbin would fit in the M Class bobbin case, but sewing with it would be impossible. The bobbin would not fit properly and would slide around in the bobbin case causing all sorts of problems.
As you might have guessed, the M bobbin holds considerably more thread than the L bobbin. You will find the larger M Class bobbin used in most longarm quilting machines and other more industrial type machines. Generally speaking, home sewing and embroidery machines use either the L Class or Class 15 bobbin. Commercial sewing and embroidery machines with vertical rotary hook bobbins primarily use the L Class bobbin and will not accept the Class 15.
A large percentage of home machines made today by Babylock, Brother, Janome and other manufacturers supply bobbin cases designed for Class 15 bobbins. This is normally the size of the empty bobbins supplied with their machines. The terms L Class and Class 15 are often used interchangeably, but as the next graphic shows they are not identical.
Class 15 bobbins and L Class bobbins differ in height. The Class 15 bobbin is approximately 1/8″ taller than the L Class bobbin. Both diameters are the same. The logical conclusion would be to assume any machine that accepts a Class 15 bobbin would also accept the regular L Class bobbin. While this is basically true, using them interchangably may not be as straight forward as you think.
Most machine manufacturers recommend winding your own bobbins using specific brands of thread on the supplied Class 15 bobbins. They claim their machines are tuned for optimum performance with these products. Whether or not this is true goes well beyond the scope of this article, but the reality is many of us can’t be bothered winding our own bobbins unless there is a specific reason for doing so. Many of us prefer to use pre-wound bobbins for cost savings and convenience (see our article on winding your own bobbins).
Since most pre-wound bobbins are L Class bobbins, something must be done to make up for the difference in height. Machine manufacturers are aware of this and normally include a center pin or spacer with the machine to make up the difference. Even when using the center pin or spacer there may be consequences. If your machine has a bobbin thread sensor to alert you when your bobbin thread is running out, using the center pin may cause the sensor to not function properly. If you do not use the center pin or spacer there is a chance your bobbin will bounce up and down in the bobbin case causing tension problems.
As you can see it’s easy to tell the difference between these three common bobbin sizes. To determine which one is best for you, run some tests with your machine to see what works and what doesn’t work. Try different sizes and brands. Then stick with the one that gives you the best results.