I tried to pop out the crank on a 79 P200. Cases split, clutch out, flipped the clutch nut over, put it on, and hit the crank end with a hammer, (hard), afterwards the nut would not come off. Knackered the threads. Damn!
I've read a bit about thread files.
I think there are enough good threads left to safely tighten the nut, if I can salvage the the ones that are knackered.