Crank sprocket rear washer/belt protector

I've just knocked my sprocket back on and was wondering what stops the rear washer from rattling around??

 

the balancer bolted on

 

I meant the one on the engine side of the sprocket!

 

The rear washer is held in place between the crank sprocket and the face of the crank where the oil seal is. As Chad said, once the crank pulley is tightened up this locks the washers axially in place.