Hello, a bolt on my bike luggage rack has become well and truly stuck,the Allen head recess has rounded off and the outer edge is smooth so no chance of grip, so I'll have to drill it out. I'm guessing it a stainless steel bolt but I'm not 100%, what type of drill bit do I need.? Also should I get a drill bit 1 or 2 mms smaller than the thread of the bolt? I have done this before but it was ages ago,cheers Jim