How to Drill Out a Broken Bolt (Step-by-Step Guide)
A step-by-step guide to drilling out a broken or seized bolt without damaging the threads — tools, technique, drill bit sizes, and extractor tips.
When a bolt snaps off flush — or below the surface — drilling it out is often the only way forward without destroying the part. Done right, you extract the broken bolt and save the original threads. Done wrong, you drill off-centre and turn a simple repair into a major one. Here’s the correct order of operations, tools and technique to drill out a broken bolt safely.
Before you drill: try the easy fixes first
Drilling is the last resort — if any of the bolt protrudes, start here.
- Soak the threads in penetrating oil for 15–30 minutes (longer for rust).
- Grip the stub with locking pliers and back it out gently.
- If the head sheared but the shank protrudes, the released tension often lets it spin out by hand.
If the bolt is flush or recessed, move on to drilling.
Tools you’ll need
A handful of basics — the left-hand bits and extractor do the real work.
- Centre punch and hammer
- Variable-speed drill (with reverse)
- Left-hand drill bits — these alone often remove the bolt
- Screw extractor set (easy-out)
- Penetrating oil and cutting fluid
- Safety glasses
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Left-hand (reverse) drill bit set
Run in reverse, these bite counter-clockwise and frequently spin a broken bolt right out — no extractor needed. The single most useful thing in a broken-bolt job.
Step by step: drilling out a broken bolt
Centre, pilot, enlarge, extract — in that order.
- File or grind the surface flat — an even top stops the bit wandering off-centre, the single biggest factor in a clean extraction.
- Centre-punch the exact middle — the divot guides the bit and stops it skating.
- Drill a pilot with a left-hand bit, in reverse — steady pressure and cutting fluid; the heat and CCW bite often grab the bolt and spin it out.
- Enlarge the hole only if needed — step up through larger bits, staying centred, and stop well before the bolt’s thread diameter.
- Use a screw extractor — tap it into the pilot hole and turn counter-clockwise; go slow, as extractors are hardened and brittle.
Screw extractor (easy-out) set
Hardened, tapered flutes bite the pilot hole and thread the broken bolt out counter-clockwise. Match the extractor to the bolt size and turn slowly — a snapped extractor is far harder to remove than the bolt.
Drill bit size guide
Match the pilot bit and extractor to the broken bolt.
| Bolt size | Pilot drill bit | Extractor size |
|---|---|---|
| 1/4" / M6 | 1/8" | #1 |
| 5/16" / M8 | 5/32" | #2 |
| 3/8" / M10 | 3/16" | #3 |
| 1/2" / M12 | 1/4" | #4 |
If the bolt is seized or rusted
Break the rust bond with heat before you drill.
Apply a propane torch to the surrounding metal (not the bolt) for a few seconds, let it cool slightly, then reapply penetrating oil — the thermal cycling draws oil into the threads. Repeat a couple of times before drilling.
Penetrating oil
A proper penetrant (not WD-40) creeps into corroded threads and, with a heat cycle or two, frees many bolts before you ever pick up a drill.
Common mistakes to avoid
Four ways a broken-bolt job goes from annoying to expensive.
- Drilling off-centre — ruins the host threads; punch carefully.
- Skipping cutting fluid — overheats and dulls bits, and work-hardens the bolt.
- Forcing the extractor — a snapped extractor is hardened steel and nearly impossible to drill.
- Drilling too large — cutting the part’s threads turns extraction into thread repair.
Frequently asked questions
Can you drill out a bolt without an extractor?
Often, yes. A left-hand drill bit run in reverse frequently grabs the bolt and backs it out on its own, which is why pros reach for them first.
What size drill bit for a broken bolt?
Start small — roughly half the bolt’s diameter — and step up only as needed. See the size chart above for common bolt sizes.
Why does my drill bit keep slipping off the bolt?
The surface isn’t flat or the centre-punch mark is too shallow. File the top flat and make a deeper punch divot before drilling.
How do I get a broken extractor out?
It’s the worst case — a snapped extractor is harder than a drill bit. Options are a carbide burr, a left-hand carbide bit, or spark-erosion (EDM) at a machine shop. Prevention beats cure: go slow and don’t force it.
Keep going
Bottom line: drilling out a broken bolt is all about staying centred. File the break flat, punch dead centre, and try a left-hand bit in reverse first — it often does the whole job. If not, step up carefully and let a screw extractor back it out. Rush the centring and you’ll trade a bolt extraction for a thread repair.