What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
Broken screws are often easier to remove with a left-hand drill bit than with a dedicated extractor. The bit cuts while turning counterclockwise, so it can loosen the screw before you drill deeply enough to use an extractor. This works particularly well on small machine screws, rusty fasteners, and screws that snapped while being driven into metal.
The best kit is not necessarily the one with the most pieces. For most repairs, you want sharp bits in useful sizes, a stiff shank that will not flex, and a drill that can run slowly in reverse. Cheap bits can be perfectly adequate for a one-time repair, but poor geometry or soft steel makes them frustrating on hardened screws.
As an Amazon Associate we earn from qualifying purchases at no extra cost to you.
What we cover
What to Look For in Left-Hand Drill Bits
Choose bits made from high-speed steel (HSS) for general work. HSS is suitable for mild steel, aluminum, brass, and most ordinary screws. A kit containing cobalt left-hand drill bits is a better choice for stainless steel, heat-treated fasteners, and repeated use. Cobalt bits tolerate heat better than standard HSS, but they are more brittle and cost more. They still need cutting oil and controlled pressure.
Look for a size range from about 1/16 inch through 3/8 inch, or roughly 1.5 through 10 mm. The smaller sizes handle stripped or snapped screws; the larger sizes give you a useful pilot hole for an extractor. A kit with every size is convenient, but a short set of common sizes is often cheaper and easier to keep sharp.
Split-point tips, commonly ground at 135 degrees, are helpful because they resist walking on a flat broken screw. A standard 118-degree point can work if you center-punch carefully. Avoid bits with a long, skinny profile when possible. They flex more easily, wander off-center, and can snap inside the fastener.
Which Type Makes Sense?
| Type | Best use | Advantages | Limitations |
|---|---|---|---|
| Standard HSS | Occasional repairs in mild steel | Low cost and easy to replace | Dulls faster with heat or hardened screws |
| Cobalt HSS | Stainless steel and repeated extraction work | Better heat resistance and edge life | More expensive and more prone to chipping |
| Left-hand bit and extractor kit | General broken-bolt and screw removal | Provides both drilling and gripping options | Included extractors may be soft or poorly sized |
| Individual premium bits | Critical repairs or frequent shop use | Consistent cutting and stronger construction | Higher cost; you must select sizes yourself |
For a homeowner dealing with a few broken screws, a left-hand drill bit extractor set is usually the practical choice. It gives you bits and extractors in one case. The cheaper option is fine when the fastener is mild steel, accessible, and not structurally important. Spend more on cobalt or premium individual bits when the screw is stainless, corroded, hardened, or buried in an expensive part.
Choose the Right Bit Size
The bit must be smaller than the screw’s outside diameter, but large enough to remove useful material from its core. Start with a bit approximately one-third to one-half the diameter of the threaded section. For a broken screw with a 1/4-inch outside diameter, a 1/8-inch or 5/32-inch bit is a reasonable starting point.
Do not begin with the largest bit that fits. If it wanders, it can damage the female threads or enlarge the hole beyond repair. Work up one size at a time if the first bit does not loosen the screw. Check the extractor instructions as well: the extractor’s required pilot hole determines the final drilling size.
How to Use a Left-Hand Bit
First remove paint, dirt, and rust from around the fastener. If the screw is flush or slightly recessed, make a firm center-punch mark. A shallow dimple is enough; a deep punch can expand the fastener and make it tighter. Clamp the work securely and wear eye protection. Broken bits and metal chips are sharp.
Install the bit with its cutting edge facing the correct direction and set the drill to reverse. A cordless drill with a variable-speed trigger works well. Keep the speed low—roughly 300 to 600 rpm for a 1/8-inch bit in steel, and slower for larger bits. Use cutting oil on steel and stainless steel. Do not use hammer mode; impact action can break the bit and damage the surrounding part.
Keep the drill square to the fastener and apply steady, moderate pressure. The bit should produce a continuous chip. If it squeals, polishes the surface, or turns blue, stop and let it cool. Excessive speed is a common failure mode. So is forcing a dull bit, which creates heat without cutting.
Drill about 1/8 to 1/4 inch deep, then pause and reverse the bit. Often the counterclockwise cutting force will catch the screw and wind it out. If the screw does not move, deepen the hole gradually. Once the pilot hole reaches the extractor’s recommended depth, switch to the extractor rather than drilling all the way through.
Avoid These Failure Modes
A broken left-hand bit is harder to remove than a broken screw. This usually happens when the bit is run too fast, tilted, or forced after it has dulled. Cobalt bits are especially unforgiving of side pressure. Keep the drill aligned and replace a bit when it stops making chips.
Do not use an extractor that is too small. It can snap under torque and leave hardened steel in the hole. Do not hammer a large extractor into a thin-walled screw either; the expansion can lock the fastener tighter. If the screw is badly seized, apply penetrating oil, add gentle heat where safe, and allow time for it to work.
For a screw broken below the surface, a guide bushing or drill guide can keep the bit centered. On an expensive engine, gearbox, or aluminum housing, drilling freehand may cause more damage than the original failure. In that situation, a machine shop or professional extraction service can be cheaper than replacing the part.
Buying Advice by Job
For occasional household repairs, buy an inexpensive HSS set with sizes from 1/16 to 1/4 inch. For automotive work or stainless fasteners, choose a cobalt reverse drill bit set and use cutting oil. If you regularly remove broken bolts, a premium left-hand bit set with replaceable individual sizes is preferable to repeatedly replacing an entire bargain kit.
The most useful upgrade is often not a larger kit but better control: a drill with a low-speed trigger, a rigid chuck, a center punch, cutting oil, and a way to hold the work securely. Left-hand bits are a simple tool, but accurate starting, low heat, and patience determine whether the screw comes out cleanly.