Even as good as I am at researching, I cannot find the inside diameter of the hole for the Improved Family bobbin. I did find the bobbin number for the bobbin is 2073 and will also work with Singer models 17, 17U, 18, 18U, 18-22, 18-35, 18-36, 18U222, 68 Class machines. I did take my calipers to my Sidewinder and found the diameter of the bobbin spindle to be .205 inch. That does not include the spring. There is a wire spring that will hold a regular size 15 bobbin which has a hole that is .245 inch in diameter. Please note the decimal point.
Janey - Neat people never make the exciting discoveries I do.