First off, that's not a crack. It's just a ridge left by a seam in the mold.

Second, you can easily repair that arm.

The arm is pinched in place and yours has not broken, it has just come loose.

Note how the inner ring has 4 indentations (Picture 1). This is where the ring was mechanically pushed out to squeeze itself against the outer ring.

Put the outer ring back in place and tap the 4 indentations on the inner ring with a punch to push them out against the outer ring and you'll be fine (just make sure the arm is in the right position first).