One reason that the foreign made ( not just Chinese ) bobbins don't always work in our older machines is because of the
dissimilarity of the measuring systems. Some times foreign makers do not use the inch measurements, they just convert it to metric and go from there. What they do a lot of the time is round up to the next largest MM and that is what throws the specs off.
If the inch measurement converts to 11.5 mm, they round it up to 12.0 mm and then it's too big.
Tammi,
There is nothing wrong with an in-spec plastic bobbin except it's plastic. My wife has been using them interchangeably with the metal ones since she got her Singer 538 back in '78. Nary a problem.
I've used them in Singer 66s made back in the 19 teens with no troubles either. The only plastic bobbin I had that didn't work was one that got dropped on the floor and stepped on. KArunch .....
Joe