"Mutter gluehen". Yes, I have had to heat those darned nuts in the past to get them loose. Lord only knows what gorilla had tightened them in the past, but it was definitely overkill. The main problem with heating those nuts is that the fiber timing gear usually catches fire or at least gets hot enough to smoke. It's going to be replaced anyway, but who needs a fire here?
Follow the advice of others and install the hex-headed nut. It will make life easier on the next guy, who has to change timing gears beside a lonely back road.
Marshall