I do this stuff for a living.
Now I’ve written previously about removing broken bolts……..pity you didn’t see it.
Before you even start, you need to equip yourself with a large selection of obscenities, profanities, blasphemies, oaths, curses, abuses and insults….. believe me you can never have enough, any shortfall in this regard will probably result in a brain aneurysm due to insufficient release of pressure!
This is an OH&S issue and no place to skimp.
Another thing to consider is appeasing the engineering gods, you will need to make a blood sacrifice, hopefully they will settle for some skinned knuckles, some bruises and a cut or two…… but you will need to be leaking blood or the job is doomed.
Now down to business…..
Rule #1, don’t attempt drilling without proper preparation first, you need to grind the broken area with a ball ended carbide burr in a die grinder, you can use a drill at a pinch.
You want to create a depression in the bolt.
Rule #2, in order to drill in the centre you need to use a centre punch to create a crater in the centre of your ground depression, be for drilling. If you miss the centre, grind the crater away with the die grinder and try again.
Rule #3 use a drill bit and drill that is properly suited to the task, a slow pneumatic drill is best, butt a good quality battery drill will do, by which I mean something that has very good balance and progressive speed control, cheap shit and handyman rubbish rarely have these qualities, Hilti and Bosch professional series drills are by far the best battery drills.
Next is the drill bit, yet again handyman rubbish won’t do, you need cobalt drills M35 (5% cobalt) or M42 (8% cobalt) with a 135 deg tip.
Rule #4, use a proper drilling cutting fluid, not rubbish like WD40!
Rule #5, first drill a 3mm pilot hole, for this I use the short double ended cobalt drill bits that are used for drilling out rivets, the reason is that they’re stronger. Then I use left hand cobalt drill bits to expand to hole, the reason for the left hand bits is that when they grab they will exert an unscrewing force on the bolt, remember to put the drill in reverse to use left hand drill bits and use a very slow speed, this will increase the chances of the bit grabbing in the bolt, work you way up in diameter, in stages so maybe for a M10 bolt start with a 5mm drill bit, then M7 and if it doesn’t come out with that then you would go to the size that you drill to tap a hole, in the case of M10 that would be 8.5mm.
Once you’ve done that the individual threads will be able to be easily removed.
Run a thread tap down through the thread of the bolt hole to clean up.
Rule #6, for small diameter bolts ( under M12) ezyouts an other bolt removal gimmicks, are a waste of time and money and are likely to break off in the bolt leaving you with a real headache, however they do work quite well in very large bolts over M30 and up to M75……not something that you’re likely to encounter in automotive use.
YOU DO NEED TO DRILL CENTRE AND STRAIGHT, and FFS don’t use crap, be properly prepared, take your bloody time and be careful……. Bugger it up and it will more than likely very costly to fix in time, effort, frustration and finances, as you are about to find out!
"Keep calm, relax, focus on the problem & PULL THE BLOODY TRIGGER"