If it is the nut described above, it is a locknut. It will only thread on about halfway before there is resistance. Put the brake rotor and caliper mounting bracket on, then slip a large screwdriver into the rotor vent bracing against the bracket so you can spin the nut on. If you had someone recut the threads in the nut, I would strongly consider replacing it. Those nuts are only supposed to be used once as it is.