Originally Posted by lewisnc100

The woodruff key is just a half circle piece of metal that ties the crank to the harmonic balancer pulley. If you mistakenly lost it the crank would turn and the harmonic balancer would not. This isn't my website, but if you go down you'll see a picture of a woodruff key. Pretty small and easy to lose, just make sure its there when you put the pulley back on and you'll be fine.

http://www.bimmerboard.com/forums/posts/16848