Broken bolt removal
Posted: Tue Apr 06, 2021 9:47 pm
The drill guide might work sometimes but usually the bolt shears just above the surface not below it.
This frequently happens on head bolts which are high tensile and need cobalt drills to touch them.
The head bolts have a long thread which is totally corroded so a left hand drill or even a top quality stud extractor has no chance.
The way I do is to file the stud flat and centre punch the stud. Clamp the block on the milling machine and drill out the stud in stages until there is only a small amount before the drill touches the thread. Care must be taken as it is difficult to keep the drill straight and true so inevitably there will be some thread left off centre. A stud extractor or even a screwdriver might then shift some of the stud, but the most successful is to use a No 1 tap (the tapered one in a set of three) of the correct thread. Use HSS or Cobalt taps not the cheapos.
I have had a few stud extractors break and also drills, which will make the job incredibly difficult as no drill will touch them as they are too hard.
Another technique is to weld a nut onto the stud. This not only gives you something to turn but the welding usually breaks the corrosion lock. Some care is needed if the stud is in aluminium but the block will be scrap anyway if you do not remove the stud!
Experience teaches you not to break studs and to know the torque they will take. Perhaps use a torque wrench set to just above the recommended torque or below the expected yield point of the corroded bolt! Preparation with heat and soaking in diesel makes it less likely to break.