Hello Julie,
The black part is actually a pin that goes througfh the unit so when you raise the presser foot it opens the tension discs, you need the return spring which any good sewing machine shop shop have along with the pin, are you sure the unit has sheered off and not just come out, if you look at the inside of the faceplate where the tension unit goes you should see a screw that holds the unit it.
kendourof, November 2010
