There is no Woodruff key. There are two dowel pins.

This bolt is supposed to get a few hundred pounds of torque so when you chase these threads, proceed carefully and get it EXACTLY RIGHT!

Even more important to point out is what I said in the first sentence. Many of these crankshafts have been ruined by someone putting it together as if it were a Woodruff key. They slap the pins in, throw the balancer on, start the bolt and run it in with an impact wrench. It would be a pretty safe bet that this is what led to the failure in the original post.

What you MUST do to begin with, set aside plenty of time to get this balancer on correctly and have a harmonic balancer puller close at hand and plan on possibly using it a number of times. Place the balancer carefully on the crankshaft snout slightly counterclockwise from pin hole alignment. Start the bolt and run it BY HAND down a good bit and then remove the bolt and see if the pin holes align. If they are close enough to seat the pins with light hammer tapping, then put the pins in place and then replace and torque the big bolt. If the holes are not aligned, pull the balancer and try again.

If you do not do this carefully, you have a great chance of destroying the crankshaft and the balancer. The balancer seems to be harder than the crankshaft. I think it is steel vs. the crankshaft being cast iron, so the balancer will probably not get hurt, it is the pins and crankshaft snout that get destroyed.

PROCEED CAREFULLY and DON'T get in a hurry.

My $0.02,