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Before they learned how NOT to, my sons broke studs off in the aluminum hubs of their race karts. I have had success removing the broken studs by building a "knob" with multiple tacks with my MIG welder, or welding a nut over the broken stud. Sometimes has taken multiple attempts but ultimately has been successful. I use a wrench on the nut, or twist the knob with vice-grips.
The Micro-Bus Bunk Bed and Playhouse.