M24 x 1.5
Go with the thread file if you can.
If you're really stuck, take an old axle nut, or preferably a skinnier nut of the same size, cut through it on one of the points. On the flat will work but not as well. Only cut through one side. Pry the nut wider until it fits over the axle threads and slide it on until it is on the other side of the damaged part. This is where the skinnier nut would be handy. Clamp it tight again and wind it off as usual. This should clean up the threads enough to use unless you really mangled them. If it starts to jam up just wind the nut on then off alternately.
Hope that helps some.
