Originally Posted by katyquilter
I'm about to try a plastic bobbin in my 99K which I recently purchased at a thrift store. It came with one metal bobbin and I bought the metal 66 bobbins which should work. they fit but do not work - the machine refuses to sew, it's as if the bobbin is just a hair too tight. So someone suggested plastic. I'm going to have to get some to try soon unless someone has a better suggestion.
Some of the new metal class 66 bobbins do not work as well as the vintage bobbins. The plastic class 66 seem to be fine. In addition, people will hang onto bent metal bobbins (which will cause problems), and will toss out broken plastic bobbins. The plastic bobbins, in gereral, will work fine in the older machines. Be sure to have the correct class of bobbin, however, and as it has been pointed out, it is often difficult to tell one from the other. I keep mine in labeled containers, and always keep at least 6 bobbins with each machine.
Please note that not all metal bobbins are equal, nor are all plastic bobbins. Each machine takes its own class of bobbins. If you try to use a class 15 bobbin in a machine that takes class 66, you will have a problem. Both of these bobbins are available in metal and plastic. There are a lot of bobbins that look alike, but are not. A class 15 plastic bobbin looks very much like a class 15J plastic bobbin, which is similar to a class 66 plastic bobbin, and so on. The classes are not interchangable.
I like to keep an index card with each machine with notes on settings for different applications, the serial number, and the bobbin class, as well as the threading direction through the needle. It's a great time saver.