When the Singer 221 Featherweight originally came out, it was very difficult to remove the bobbin case base from the rotary hook. You had to take the hook off the machine and remove the safety spring on the back to get the gib arm to swing out and release the bobbin case base.
Singer quickly redesigned the rotary hook and during the second production run of Featherweights in 1934, the second style rotary hook became standard. This hook was then used on all Featherweights until the end of production.
The later style rotary hook allows for easy removal of the bobbin case base to remove a thread jam, clean, and oil the hook.
We offer the later style rotary hook, complete with gib arm, screw, bobbin case base, and both set screws for attachment to the hook shaft.
After installing a new hook, when you put the needle plate back on don't forget to align the position finger of the bobbin case base to the twelve o'clock position, in between the leaf springs on the bottom of the needle plate!