A drill extractor (also called a screw extractor) removes broken bolts, screws, and studs by gripping the inside of a pilot hole you drill into the damaged fastener, then turning it counterclockwise to back it out. The process has a few critical steps: preparing the surface, drilling a centered pilot hole, choosing the right extractor size, and applying slow, controlled torque. Get any of those wrong and you risk snapping the extractor inside the hole, which makes the job significantly harder.
Types of Extractors
There are three main styles, and each works best in different situations.
Straight fluted (four-sided taper): A tapered piece of hardened steel with flat sides. You hammer it into the pilot hole, and the taper wedges against the walls to grip. These are the cheapest option and work well in soft materials like aluminum or metals with a hardness below about Rockwell C 15. They’re the most basic extractor you’ll find in hardware store kits.
Spiral fluted (tapered): These look like a reversed drill bit and screw into the pilot hole rather than being hammered in. You turn the square head with a wrench or socket, and the spiral bites into the hole as it advances. Spiral extractors are better for harder metals because they’re manufactured at a higher hardness. The tradeoff is that harder steel is more brittle, so these can snap without warning if you apply too much torque.
Multi-spline (with a turn nut): These have six narrow splines that cut into the inside of the pilot hole, providing more contact area than either of the tapered types. The design distributes force more evenly, resists breakage better, and allows rotation in both directions so you can rock the fastener back and forth to free it. They cost more, but they’re the most effective option for stubborn or high-strength bolts.
Choosing the Right Size
Matching the extractor and pilot drill bit to the bolt diameter is essential. A hole that’s too small won’t let the extractor grip; too large and you’ll damage the threads in the surrounding hole. Most extractor sets include a sizing chart, but here are the common pairings for spiral extractors:
- 3/32″ to 5/32″ bolt (No. 12 screw, 2.5–4 mm): 7/64″ pilot hole, #2 spiral extractor
- 7/32″ to 9/32″ bolt (6–8 mm): 5/32″ pilot hole, #3 spiral extractor
- 9/32″ to 3/8″ bolt (8–10 mm): 1/4″ pilot hole, #4 spiral extractor
- 3/8″ to 5/8″ bolt (10–16 mm): 19/64″ pilot hole, #5 spiral extractor
- 5/8″ to 7/8″ bolt (16–22 mm): 13/32″ pilot hole, #6 spiral extractor
- 7/8″ to 1-1/8″ bolt (22–27 mm): 17/32″ pilot hole, #7 spiral extractor
For straight fluted extractors, the sizing is slightly different. A 3/8″ bolt typically uses a 3/16″ pilot hole with a #2 straight extractor, while a 1/2″ to 9/16″ bolt uses a 5/16″ pilot hole with a #4 straight. Always check the chart that came with your specific set rather than guessing.
Preparing the Broken Fastener
Before you pick up a drill, apply penetrating oil to the broken fastener and the surrounding area. Penetrating oil needs at least 15 minutes to work its way into the corroded threads, and heavily rusted bolts can benefit from soaking for 30 minutes to an hour. For especially stubborn cases, apply a second round and wait again. This step alone can make the difference between a clean extraction and a snapped extractor.
If the bolt broke off flush or below the surface, use a carbide burr or grinding tool to flatten the top so you have a clean, level surface to work with. Then use a center punch and hammer to mark the exact center of the broken fastener. This dimple guides your drill bit and prevents it from wandering. Getting the pilot hole dead center is the single most important factor in a successful extraction. An off-center hole weakens one side of the remaining bolt wall and can cause the extractor to break through into the surrounding threads.
Drilling the Pilot Hole
Start with a small 1/8-inch drill bit regardless of the final pilot hole size you need. This narrow starter hole is easier to keep centered and acts as a guide for larger bits. Drill slowly, with steady downward pressure, and clear metal chips frequently by backing the bit out.
Once the starter hole is in, step up to the pilot hole size specified for your extractor. If there’s a big jump between 1/8″ and your target size, use an intermediate bit to keep things accurate. Drill to a depth slightly deeper than the length of the extractor’s tapered section so it has room to seat fully.
Using Left-Handed Drill Bits
Left-handed (reverse) drill bits spin counterclockwise, which is the same direction you’d turn to loosen a standard bolt. Set your drill to reverse and use a left-handed bit for the pilot hole. As the bit cuts into the broken fastener, its counterclockwise rotation sometimes catches and backs the broken piece right out, solving the problem before you even need the extractor. This works especially well on screws that are stuck but not heavily corroded, or on stripped screws where the head slots are destroyed.
Inserting and Turning the Extractor
For a straight fluted extractor, place the tapered end into the pilot hole and tap it firmly with a hammer until it seats snugly. For a spiral fluted extractor, insert the tip into the hole and turn it counterclockwise by hand until it begins to bite.
Attach a T-handle tap wrench or a socket wrench to the square head of the extractor. A tap wrench gives you the best feel for how much torque you’re applying, which matters more than you might think. Turn counterclockwise slowly and steadily. You should feel resistance as the extractor grips, then the broken fastener should begin to rotate. If you’re using a multi-spline extractor, you can rock it clockwise and counterclockwise to break the corrosion bond before committing to full counterclockwise removal.
Apply more penetrating oil as you go. If the fastener starts moving and then seizes again, back the extractor slightly clockwise, add oil, wait a few minutes, and try again.
Avoiding a Broken Extractor
A broken extractor stuck inside a broken bolt is one of the worst outcomes in fastener removal. Extractors are made from hardened steel that’s extremely difficult to drill through, so preventing breakage is critical.
The most common cause is too much torque. Smaller extractors, like a #4 spiral, can snap at surprisingly low force levels. Carbide extractors are especially unforgiving: they feel completely solid right up until they fracture, with no gradual bending to warn you. If you feel significant resistance and the fastener isn’t budging, stop. Apply heat with a propane torch around the surrounding material (not directly on the extractor) to expand the hole slightly, add more penetrating oil, and try again after it cools.
Other ways to reduce breakage risk:
- Never use an impact driver on a screw extractor. The sudden hammering force can snap it instantly.
- Keep the extractor aligned straight with the hole. Side loading concentrates force on one edge of the extractor.
- Use the largest extractor that fits. A larger cross-section handles more torque before failing.
- Choose multi-spline extractors for high-strength bolts. Their design distributes torque more evenly and resists breakage better than spiral types.
When Standard Extraction Fails
If the bolt won’t turn even with a properly seated extractor, you have a few options before giving up. Applying heat to the base material around the bolt (a few cycles of heating and cooling) can break the corrosion bond that penetrating oil alone couldn’t reach. On steel bolts in cast iron or aluminum, the difference in expansion rates between the two metals helps crack the rust seal.
Welding a nut onto the top of the broken bolt is another approach. The heat from welding helps loosen corrosion, and the nut gives you a solid hex surface to grip with a socket wrench, letting you apply far more torque than any extractor can handle. This only works if the bolt is accessible and you have welding equipment, but it’s often more reliable than extraction for large, heavily seized fasteners.