Prasinos,
Not questioning you, well, maybe I am, but are you sure you got that right? I've looked at a number of these engines and see the opposite: Fine threads, actually 8mm X1 in the crankcase and 8mm X 1.25 on the top or nut end. I was going to replace the studs with bolts but was unable to locate an 8mm X 1 bolt long enough. When they get to that length they go to a 1.25 thread. I could find shorter bolts in a 1 thread but not longer ones. I even tried a place in Denver that specialize in metric fasteners.
While we're talking about this another thing I discovered while 'tinkering' was that some of the acorn nuts were bottoming out on the studs before they were tight against the cylinder head. That wouldn't normally present a problem unless the stud was already screwed all the way into the crankcase. Then you'd have a loose head which I suspect is the reason for some of the head gasket failures I read about here. I replace all the acorn nuts with shouldered (a wide surface) hex nuts to eliminate that possibility. I leave the head off, double nut the stud and torque it into the crankcase to the recommended 12 foot pounds. Then I remove the double nuts, install the head and gasket and torque the nuts to about 10 ft. pounds. Seems to work for me.
Here's a link to my original thread regarding this issue.
Acorn vs. Hex Nuts
Tom