I also work hardens the bolt.
Robinson helicopter rotor head hardware is stretched, not torqued.
Not only that, the nut head is drilled and cotter pinned after the proper stretch is achieved. Since the bolt work hardens it can only be stretched 2 or 3 times before the drilled holes will no longer match up on the nut.
Then you have to use new hardware.
Bolt stretching is nothing new, the main crank assembly on radial engines required stretch back in the day.
