You use the tool like this.  If the pin is in there, the tip of the tool will hit the pin.

That is horrifically impressive. Makes my tank bracket failure look like a paper cut compared to losing a limb.
Best of luck with whatever happens.

That sounds like a hair raising experience.  I would really hate to have that happen on the freeway.  It could have hurt you bad.  Glad you didn't hit the pavement.

Sure would be helpful if you could comment on your primary drive nut and also do a conclusive check on the balance drive pin.  The absence of the drive pin sticking out of the crank does not conclusively prove the pin isn't in there.  You need to stick something in the hole and see if the wire or whatever goes in more that 1/2".  That info would help all of us and possibly prevent someone else from having a similar experience.  You had to have removed that nut when you did your timing chain job.  Did you possibly overtighten that nut?  For instance, did you possibly mistake the nut for having normal right-hand threads and try to remove it by turning counter-clockwise? ......or....... Did you use an impact wrench (rattle wrench) on the nut?  Please help us out Celeste.

This is the nut in question.

The Tear down has so far not yielded insight into the failure.

As you can see, the Cam Chain and sprocket are intact and still functioning.

Celeste, thanks very much for those photos.  You have a very interesting failure.  First time I have seen one that broke the balancer driven gear.  That gear is broken right in half.  Wow!

It's great that your cam drive sprocket is not fractured, but it's also not so great.  If you had a sprocket failure, at least we would know how to deal with it (i.e. pay attention to properly tightening the primary drive nut).  But if your nut was properly tightened, and the drive pin is still in the drive gear, then what caused that driven gear to fail.

If the driven gear failure is what caused the collision between the balancer and connecting rod, there really is no simple way to monitor things.  To inspect the driven, gear you have to pull the whole engine apart.  It doesn't lend itself to periodic inspection.  There are six springs in that driven gear assembly.  Every other spring should have a pin nested inside the spring.  Those pins are hard stops intended to ensure that the balancer timing does not exceed a predetermined range (so the rod doesn't hit the balance weight).  Are all three pins accounted for?  Is it possible that someone in the past assembled the gear without those stop pins?

What came first, the chicken or the egg.  Did the driven gear fail and result in the collision, or did the drive pin or drive key fail and cause the collision which caused the driven gear failure?

I can see from your picture that the primary drive nut has been beat up pretty good.  Somebody was struggling with that nut to either get it loose or tighten it up.  The sprocket looks OK, but who knows if it was replaced at some point in the past.  Operation with the nut loose or with the sprocket fractured (or both) will work the drive pin loose over time.  

Can't wait to hear and see about your drive pin.  If that drive pin is still in the drive gear, then it kinda looks like the driven gear failed and set the system up for a collision.

Please keep those pictures comin.  I really appreciate you sharing.

Mechanic Error? Perhaps.

One thing is unusual.
The Bearing between the crankcase and the counter balance gear was so tight it took two people loosen it.

The counter balance shaft itself is bent  by 5 degrees horizontally and twisted by about 2 degrees. This has to be the original point of failure.