These crank pulleys sometimes want to take a walk for unknown reasons. From what I've read it because the pulley bolts shear. Kind of has me worried myself. Doesn't seem to be any way to prevent it. Hasn't happened to me yet, but it has to a lot of others. Fun part about it is getting the balancer/pulley assembly aligned properly when you put it back together. There are round metal dowels on either side of the crank, make sure you still have yours. They're easily damaged when the whole thing is reassembled. Hopefully the crank nose is not damaged.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)