Reproduction motor cap for Featherweight 221. This is the motor cap screw that measures 3/8" across the bottom*. It covers the carbon brush inside the motor. There are actually two motor caps on your Featherweight motor, because there are two carbon brushes - one is seen clearly on the top of the motor, but the other one is hidden on the underside. You can access the bottom motor cap by inserting a screwdriver into the allotted hole on the bottom of the machine or by removing the motor altogether.
You can see in the photo above how the broken one needs replaced.
* If your motor cap is larger and measures 1/2" across the bottom (more common to White 221K and 222K Free-Arm Featherweight motors) there is not a replica part available. However, we might have an original bakelite replacement in some of our vintage parts inventory. Contact us for more details and inquiries.