sharing some great information and tips from Sewing Parts Online they are great with information that helps us all
The Class 15 (A Style) Bobbin:
The Class 15 is about the size of an American nickel. Its diameter measures approximately 20.3 mm and has a width of approximately 11.7 mm. This bobbin has two flat sides and is available in both plastic and metal.
The L Style Bobbin:
The L Style is about the size of an American nickel. Its diameter measures approximately 20.3 mm and has a width of approximately 8.9 mm. This bobbin has two flat sides and is available in aluminum, plastic, and as a Magna-glide core.
It’s worth noting that the L Style bobbins are the same diameter as the Class 15 bobbins. As such, you can use L Style bobbins in a sewing machine that uses Class 15 bobbins. However, a Class 15 bobbin is too wide to fit in a machine that uses L Style bobbins.
The M Style Bobbin:
The M Style is about the size of an American quarter. Its diameter measures approximately 24.9 mm and has a width of approximately 10.7 mm. This bobbin has two flat sides and is available in metal, and as a Magna-glide core. The Singer 8228 Bobbin:
The Singer 8228 is probably my favorite bobbin because it looks so cool. Its diameter measures approximately 9 mm and has a width of approximately 33.4 mm. This bobbin is only available in metal.
This bobbin is used in the old Singer treadle machines and fits inside a bullet-shaped bobbin case. Pretty neat!
Fil-Tec Magna-Glide Cores:
Fil-Tec’s Magna-glide pre-wound bobbins are essentially just a bobbin barrel wound with thread. The magnetic core prevents backlash, creating consistent stitches without needing a backlash spring. Once empty, the core can be disposed or recycled. Bobbin and Bobbin Case Care:
Most importantly, you must take good care of both your bobbin and your bobbin case. The condition of your bobbin and your bobbin case dramatically affects your stitch formation. It’s important to intermittently inspect both your bobbin and your bobbin case to ensure no scratches or burrs have developed. Even the slightest nick will cause skipped stitches and thread nests. We recommend gently gliding your finger along all sides of your bobbin and bobbin case to check for inconsistencies. If you do discover a small scratch or burr, you should replace you bobbin immediately. A small scratch on your bobbin case can usually be gently buffed out with fine sandpaper. However, serious damage requires buying a whole new bobbin case.
Some longarm machines (A-1, Gammill, and Handi Quilter) take the M-style bobbin, which is larger in diameter than both L-style and Class bobbins. L-style bobbins and Class 15 bobbins are nearly identical in diameter (Class 15 bobbins are about 0.5 millimeters larger in diameter). L-style bobbins used to be the most popular bobbin style but with the abundance of rotary bobbin mechanisms and drop in bobbins, the Class 15 bobbin style is also a very popular size.