What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
A left-hand drill bit can remove a broken bolt while it drills, because its cutting direction is opposite the direction used to tighten most fasteners. If the bit catches, it may turn the bolt out before you need an extractor. It is not a guarantee: a seized bolt, a badly off-center hole, or a bit that breaks can make the repair harder. Work slowly, keep the drill square, and stop if the bit wanders.
What we cover
When a left-hand bit is the right choice
Try this method when a bolt has snapped near the surface and you can reach it with a drill. It works best on a bolt that is not heavily corroded or bonded in place. A broken fastener recessed deep in a hole, or one that snapped below the surface at an angle, is a poor candidate for hand drilling.
Left-hand bits are usually made from high-speed steel (HSS) or cobalt steel. HSS is adequate for ordinary mild-steel bolts. Cobalt bits cost more but hold up better in harder alloys and when heat is difficult to control. A basic left-hand drill bit set is useful because the right pilot size matters more than having one large bit.
Tools and preparation
You will need a variable-speed drill, a center punch and hammer, cutting oil, safety glasses, and a bit sized for the bolt. A cordless drill is fine if it has a controllable low-speed trigger and a sound chuck. Do not use an impact driver: its hammering action makes it harder to keep the bit aligned and can snap a small bit. For a bolt around 1/4 inch (6 mm), begin with a pilot bit roughly 1/8 inch (3 mm); check the bolt’s thread size and an extractor chart before drilling larger.
If the broken end is uneven, file or grind it flat enough to mark the center. Wear eye protection, clamp or secure the work where possible, and keep loose clothing clear. If the part is attached to a vehicle or machine, disconnect power and avoid drilling where chips can enter bearings, wiring, or fuel components.
Drill the pilot hole
Apply penetrating oil around the bolt and give it time to work. Mark the center with a punch. A centered start is critical: if the hole breaks through near the bolt’s threads, you can damage the surrounding part and leave little material to grip.
Fit the left-hand bit with its flutes cutting as the drill rotates counterclockwise, viewed from the chuck end toward the work. Confirm the drill is set to reverse. Use the lowest practical speed—often about 300–600 rpm for a small steel bit—and light, steady pressure. Keep the drill perpendicular to the broken face. Add cutting oil, drill a short distance, then withdraw the bit to clear chips. Do not force a hot bit; excess heat dulls the cutting edge and makes a small bit more likely to snap.
Drill to a depth appropriate for the bolt, not all the way through the workpiece. A piece of tape on the bit can serve as a rough depth marker. If the bit skates, stop and re-center rather than trying to steer it under power.
What to use if the bolt does not turn
| Method | Best use | Main drawback |
|---|---|---|
| Continue with left-hand bits | Bolt begins to loosen as the pilot deepens | Slow; larger bits need accurate centering |
| Screw extractor | Centered pilot hole, bolt still firmly stuck | Can break if driven too hard; hardened fragments are difficult to drill |
| Drill out the bolt | Extractor is unsuitable or the bolt is already damaged | Risk of damaging internal threads |
If the bolt starts backing out, keep the drill in reverse and reduce pressure. If it does not, stop before enlarging the hole. Choose a screw extractor sized for the pilot hole and follow its instructions; many extractors require a specific drill size. A spiral extractor can grip well, but too much turning force can wedge it tighter. Turn it gently with a tap wrench, not a powered drill. A screw extractor set is inexpensive for occasional repairs, but quality and size coverage vary.
Avoid common failures
The most costly mistake is snapping the drill bit or extractor inside the bolt. Small bits are brittle, especially when bent sideways; hardened extractor steel is harder still. Keep the drill aligned, use cutting oil, clear chips, and do not use a wrench to apply excessive torque. If the extractor will not move the bolt with moderate hand pressure, stop and reassess.
Heat can help loosen a corroded fastener, but it is not appropriate near seals, paint, fuel, plastic, or flammable materials. On a removable metal part, controlled heating around the bolt may help after oil has had time to work; do not heat an area soaked with penetrating oil. If the bolt is seized and the part is valuable, a machine shop can drill it on a drill press with better alignment than a handheld setup.
After removal, inspect the threads. Run a matching tap through them only if needed, and avoid cutting fresh threads into the surrounding material unnecessarily. Replace the bolt with the correct grade and size, and use anti-seize only where the application and manufacturer allow it.