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@Rik_Van_Looy
If it screws in halfway you shouldn't have too much trouble.
The trickiest part of hole tapping is getting started without wonkiness.
Unfortunately the stakes are rather high, cock up and it's a brazing job....
Once you get going, do 2 turns clockwise, 1 anticlockwise to clean the swarf from the cutting edge.
put some oil in the hole before you get going, and when you get to the bottom don't keep turning or you may risk snapping the tap off in the boss!
Before resorting to the tap. a first step would be to make sure the thread is clean and the screws well lubed. Shine a torch into the thread and see if there's any crap in there, if so, hook it out with something. A bit of grease on a well cleaned screw might make matters easier.
Depending on how much thread engagement you have an alternative and rather more simple solution might be to chop the bolt short.... I've heard it stated that the minimum engagement for steel on steel should be 65% of nom. diameter (i.e. for an M6 screw that would be 4mm) so 100% should be fairly safe i.e. cut the canti fixing screw to 7mm long.