This simple plate that runs across the top of the shuttle retainer ring is necessary in order to make the machines sew properly in reverse. Unfortunately, the needle will infrequently be bent sufficiently to actually impact the edge of the opening in this plate. When that happens a burr is created that can cause skipped stitches and broken thread (especially in reverse). Generally it is possible to smooth up the rough edges of the hit and continue sewing. But it is nevertheless a good idea to have a spare cap spring or two in case of really serious damage. Fortunately, the cap spring is not too expensive nor are they hard to replace (they are held in place with two small screws). Sold Each.