try to find a metric bit. for a 7/26-20 thread up front i used a 14mm drill bit to make it very VERY tight on the knurled part of the stud, my rears are 5/8 and they screw in so i can't really help there, but a metric drill bit might be your best bet. just my .02
edit... one more thing, the drill bit may seem like its not cutting, but most likely where you are drilling heated up so much it kinda "heat treated" the metal. get a carbite type bit (what you would use for porting heads) and take a little bit of metal out with that and the drill bit will most likely start cutting again... at least that's what worked for me after buying a a 150 dollar drill doctor,