Because the bobbin is part of the hook mechanism, not an accessory hung off it. Its diameter, height and side profile are all designed around the machine, and a bobbin of the wrong class sews badly even when it physically fits.
The ones you will meet. Class 15 and Class 66 look almost identical at a glance but differ in profile — one has flat sides, the other slightly domed. Swap them and the bobbin binds or rocks in the case. L-class bobbins are smaller and used by some machines and longarms; M-class are considerably larger, holding more thread so you stop less often.
Material matters as well as class. Metal and plastic bobbins of the same class do not weigh the same and do not spin up or slow down the same way, and machines are specified for one or the other. A metal bobbin in a machine designed for plastic can wear the case over time; a plastic one where metal is specified can flex under tension.
What the wrong bobbin looks like. Tension that will not settle no matter what you do to the dial. Stitching that is fine for a while then changes. A rattle or buzz from the bobbin area. Thread caught under the bobbin or wrapped round the spindle.
Practical rules. Check the manual for the class and write it somewhere you will find it. Buy bobbins from a source that states the class, not ones described as fitting most machines. Do not mix unknown bobbins into your set. Wind on the machine that will use them.
And inspect them. A chipped or burred edge catches thread and produces a fault that looks exactly like a tension problem.
Questions people actually ask
Can I use a bobbin that fits but is not the right class?
It will sew, badly and unpredictably. It is one of the hardest faults to diagnose precisely because the machine appears to be working.
Why do some bobbins have holes round the edge?
On some machines the holes are part of how the case grips or how tension is applied, and on others they are just moulding. Match the class rather than the appearance.
Last reviewed 2026-08-11