Oh dear. I cant remember offhand. I think its 12.9. Might be 12.7. Best bet is to measure the shank with a vernier and use a drill thats about 0.2-0.3mm smaller. The other way would be to measure the low point of the splines, as opposed to the high point. Make sense? I will go measure it now.

If you have drilled it too big, it won't be a problem as long as its not sloppy. You can put a blob of weld on the head of the stud to stop the stud from turning in the hub. Its not unsafe as the head stops it from pulling through.