the tip by pelicanwheel is good,
give it a try before you blister your fingers some more, an old screwdriver with a sharpened end like a chisel works the same also, just be patient with the tapping technique and get a good angle on the screw head,
I had a similar problem on mine two days ago installing the headers...got to the last screw and it only went in two turns then stopped....
after cleaning out the thread, trying another screw, more wd40 and lots of swearing, I had to retap the existing thread...
what a pain, but I did not need to helicoil or anything like that..