The key to achieving a nice looking stitch is proper thread tension, which is nothing more than balancing the bobbin thread tension with the top thread tension. Proper tension starts with setting the bobbin case tension to the correct value, and then adjusting the top tension to match. The correct starting value on the Singer 221/222 Featherweights and the Singer 301 which shares the same bobbin case, is to set the bobbin tension to 23 grams.
Different weight threads, different manufacturer's threads, and even different color threads from the same manufacturer can require a bobbin case adjustment to start with the proper tension.
This simple tension gauge makes it easy for even the novice user to perfectly adjust their bobbin case to the 23 gram starting point.
This gauge can also be used with other model machines by simply attaching to the thread and pulling the thread out of the bobbin case. Will work with most machines, with the bobbin case in or out of the machine.