M20-2 is one of the two normal fine pitch threads for metric, used in Europe and UK. The other fine pitch is 1.5 which I think is more common in Japan, China etc
Some dingbat in CIH thought it would be cool to spec M20-2.0 for the holes under the transmission housing.
Actually, this tractor hasn't started the season yet. The clutch took a dump last winter and I have finally started on it. Will need it in 6 weeks or so.
I suppose but
1) why not just cut the bore oversize in the first place as the bearings to do so are readily available and the bore will have to be re cut anyway if it is welded up and
2) I'm not sure it will work to weld a bore in an iron housing.