As an enduro racer with a kart with stubs that have done many, many hours, I'm interested in this. In fact, my stubs are on their 4th frame.
My gut feeling is the same as Mark's - they don't need changing. I think this because if you look at the front corner of a kart, it looks to me that the king pin is by far the weakest point - it's further from the tyre so there is more leverage on it, yet they are usually less 'stout' than the rest of the stub.
I've had karts with m8 kingpins that did fail. They are generally m10 or bigger now, but I still like to change them every now and again - fairly cheap.
In oekc, I've not noticed any stubs breaking. I think the whole yoke breaking off the chassis is more common.
I probably should point out that I'm not an engineer, so would be comfortable with someone saying I'm completely wrong.
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