Difficulties installing & torquing rear drum - Help!

[psogden]

Am attaching a photo of the end of the axle where the threads on the 5/8 -18 failed at around 160 foot-pounds. I have removed the 5/8” washer to expose the edge of the drum. As you can see, (bit a bullet) there are still good threads on the axle. This would appear to confirm that the bolt was not a Grade 8 quality or it would not have failed. I do notice that the end of the key way on the axle is showing (right side of axle). Should the key way be exposed? Is it possible that the drum is on too far? If I understand the theory of the drum to axle fit, seemingly it would be impossible to torque the drum on too far. Am I correct or--??