When a bolt head snaps off inside a hole, extracting it is difficult. A left-hand drill bit drills a hole into the broken bolt while running the drill in reverse. As the bit cuts into the metal, the counter-clockwise rotation applies torque to the bolt. Often, the friction of the drilling action will grab the bolt and spin it right out without needing a separate extractor. Use plenty of cutting oil and keep the drill speed low. If you run the drill in forward, you will ruin the cutting edge of the bit.
Mechanic Glossary
Left-Hand Drill Bit
A drill bit with reverse flutes designed to run counter-clockwise, drilling into broken bolts to back them out of their threads.
Frequently Asked Questions
You likely ran the drill in forward. Left-hand bits are designed to cut only when the drill is running in reverse/counter-clockwise.
Yes. Always punch a starting dimple exactly in the center of the broken bolt to prevent the drill bit from walking into the threads.
Run the drill slowly with steady downward pressure. High speed generates heat that will dull the bit and harden the bolt.
Yes. Just ensure the drill is switched to the reverse setting before starting.