Drill Bits For Broken Bolts – Professional Techniques For Successful

ByJim Boslice Metalworking & Fabrication

The most effective drill bits for broken bolts are left-hand cobalt bits, which rotate counter-clockwise to help loosen the fastener while drilling. For hardened bolts (Grade 8 or higher), use M42 cobalt or carbide-tipped bits combined with a high-quality cutting fluid to prevent overheating.

Always start by creating a deep dimple with a center punch to ensure the bit stays centered, as an off-center hole can permanently damage the surrounding threads.

We have all been there—you are applying just a bit too much torque to a rusted fastener when you hear that sickening snap. Suddenly, a simple repair job turns into a stressful afternoon of staring at a headless stud buried deep inside a workpiece.

Don’t panic or reach for the sledgehammer just yet, because choosing the right drill bits for broken bolts can turn a potential disaster into a minor speed bump. With the correct tools and a patient approach, you can extract almost any snapped fastener without damaging the expensive parts they are stuck in.

In this guide, I will walk you through the specific types of bits you need, the physics of why they work, and the step-by-step shop techniques I use to clear out broken hardware. Whether you are working on a classic car, a piece of heavy machinery, or a home DIY project, these methods will help you regain control of your workshop.

Understanding the Best Drill Bits for Broken Bolts

When a bolt snaps, it is usually because it was seized by rust, over-torqued, or physically deformed. Standard high-speed steel (HSS) bits that you find in a typical home center kit are often insufficient for this task.

Most automotive and structural bolts are “graded,” meaning they are heat-treated for strength. If you try to use a cheap bit on a Grade 8 bolt, the bolt will actually dull the bit before you even make a dent.

To succeed, you need specialized drill bits for broken bolts that are harder than the fastener itself. You also need to consider the direction of the drill’s rotation, which plays a massive role in the extraction process.

The Magic of Left-Hand Drill Bits

If you only buy one specialized tool for your extraction kit, make it a set of left-hand drill bits. Unlike standard bits that cut while turning clockwise, these cut while rotating counter-clockwise.

As the bit bites into the metal, the torque is being applied in the same direction required to unscrew the bolt. Frequently, the heat and vibration of the drilling process will cause the broken stud to catch on the bit and simply spin right out of the hole.

Cobalt vs. Carbide: Picking the Right Material

For most DIY and automotive applications, M35 or M42 cobalt bits are the gold standard. They contain 5% to 8% cobalt, which allows them to maintain their hardness even when they get extremely hot.

If you are dealing with a drill bit or a tap that has broken off inside the bolt, you may need to step up to solid carbide. Carbide is incredibly brittle but extremely hard, making it one of the few materials capable of drilling through other hardened tools.

The Critical Importance of Surface Preparation

You cannot simply point a drill at a jagged, broken bolt and expect a good result. If the bit wanders off-center, you will drill into the threads of the parent material, which often leads to a much more expensive repair involving Helicoils or oversized tapping.

Start by grinding the top of the broken bolt as flat as possible. A small die grinder or a Dremel tool with a stone bit works perfectly for this, providing a level surface for your starting point.

Using a Center Punch

Once the surface is flat, you must use a center punch to create a physical divot in the exact center of the bolt. This dimple acts as a guide for the tip of your drill bit, preventing it from “walking” across the surface.

If the bolt is snapped off deep inside a hole, use a transfer punch that matches the diameter of the hole. This ensures the punch hits the absolute dead center of the broken fastener.

Lubrication and Cooling

Never drill dry when extracting a bolt. Use a high-quality cutting fluid or penetrating oil like PB Blaster or Liquid Wrench to keep the friction down.

The oil carries heat away from the cutting edge and helps the flutes of the bit clear out metal shavings. If you see smoke, you are going too fast or using too little oil, which will quickly ruin your drill bits for broken bolts.

Step-by-Step Guide to Bolt Extraction

Now that you have the right tools and the surface is prepped, it is time to start the actual removal process. Patience is your most valuable tool here; rushing leads to broken bits and ruined workpieces.

  1. Select the right size: Choose a bit that is roughly half the diameter of the bolt. If the bit is too large, you risk hitting the threads; if it is too small, your extractor tool might snap.
  2. Set your drill to reverse: If you are using left-hand bits, ensure your drill is in the reverse setting. It sounds obvious, but it is a common mistake that prevents the bit from cutting.
  3. Apply steady pressure: Use a low RPM (revolutions per minute) and high downward pressure. You want to see long, curly metal ribbons coming out of the hole, not fine dust.
  4. Watch for movement: Keep a close eye on the bolt. Often, the left-hand bit will grab the bolt and it will begin to unscrew before you even finish the hole.
  5. Increase bit size if needed: If the bolt doesn’t budge, step up to a slightly larger bit. This thins the “wall” of the broken bolt, relieving the outward pressure against the threads.

Using Screw Extractors (Easy-Outs)

If the drill bit creates a clean hole but the bolt is still stuck, you will need a screw extractor. These are tapered, reverse-threaded tools that you tap into the hole you just drilled.

Once the extractor is seated, use a tap handle to slowly turn it counter-clockwise. Avoid using a power drill with extractors, as they are made of very hard, brittle steel and will snap if subjected to sudden impacts.

Common Pitfalls to Avoid in Metal Extraction

The biggest mistake beginners make is using too much speed. High speed creates friction heat, which can actually “work-harden” the bolt, making it nearly impossible to drill with standard tools.

Another common issue is failing to keep the drill perpendicular to the workpiece. If you drill at an angle, you will eventually exit the side of the bolt and destroy the internal threads of the hole.

Dealing with Broken Extractors

If you snap a screw extractor inside the hole, you are in a difficult spot. Extractors are harder than most drill bits for broken bolts, meaning you cannot easily drill them out.

In this scenario, you may need to use a solid carbide burr or a specialized “rescue bit.” These are designed to pulverize hardened steel, but they require a very steady hand and high-speed rotary tools.

Heat and Vibration

If a bolt is truly seized by rust, don’t be afraid to use heat. Using a propane or MAPP gas torch to heat the surrounding metal can cause the hole to expand slightly, breaking the bond of the rust.

Combine heat with a few sharp taps from a hammer and punch. This vibration helps the penetrating oil work its way into the threads before you begin the drilling process.

Essential Safety Practices for Metalworking

Drilling into hardened steel produces sharp, hot metal shards that can fly several feet. Always wear ANSI-rated safety glasses or a full face shield when performing an extraction.

Keep your hands away from the spinning chuck of the drill. If the bit catches, the drill can kick back violently, which is why using a side handle on your drill is highly recommended for larger bolts.

Handling Chemical Fumes

If you are using penetrating oils and heat simultaneously, be aware of the fumes. Some older formulas can release toxic gases when heated to high temperatures.

Always work in a well-ventilated area or use a shop fan to move the air away from your face. If you are working on a vehicle, ensure there are no fuel lines or flammable materials near your heat source.

Selecting the Right Drill for the Job

While a cordless drill is convenient, a drill press is the superior choice for bolt extraction if the workpiece is portable. A drill press ensures the bit stays perfectly vertical and allows for precise pressure control.

If you must use a handheld drill, choose one with a brushed or brushless motor that has excellent low-speed torque. High-speed “DIY” drills often lack the “grunt” needed to drive a cobalt bit through hardened steel at low RPMs.

Impact Drivers vs. Drills

Never use an impact driver with drill bits for broken bolts. The hammering action of an impact driver will shatter cobalt and carbide bits instantly.

The only time an impact tool should be near a broken bolt is if you are using an impact extractor socket on a stud that is still protruding from the surface. For internal drilling, smooth, consistent torque is the only way to go.

Frequently Asked Questions About Drill Bits for Broken Bolts

What is the difference between a left-hand bit and a regular bit?

A regular bit cuts while turning clockwise (to the right). A left-hand bit cuts while turning counter-clockwise (to the left). This reverse action helps unscrew a broken bolt while you are in the process of drilling into it.

Can I use a masonry bit for broken bolts?

Generally, no. Masonry bits use a different tip geometry designed for “hammering” through stone and concrete. However, in an absolute emergency, a sharpened carbide-tipped masonry bit can sometimes grind through a hardened bolt, but it is not the ideal tool.

Why did my drill bit get dull immediately?

This usually happens because the drill speed was too high or the bolt is harder than the bit. If you are using standard HSS drill bits for broken bolts on a Grade 8 fastener, the bit will melt almost instantly. Switch to a cobalt bit and slow down your RPMs.

Is it better to drill all the way through the bolt?

Yes, if possible. Drilling all the way through relieves the internal tension of the bolt and allows penetrating oil to reach the threads from the back side. It also ensures your extractor has plenty of “bite” along the entire length of the hole.

What should I do if the bolt is flush with the surface?

If the bolt is flush, your best bet is to use a center punch and a left-hand cobalt bit. If you have a welder, you can also weld a nut onto the top of the flush bolt, using the heat of the weld to break the rust loose while providing a new head to turn.

Final Thoughts on Successful Extraction

Mastering the use of drill bits for broken bolts is a rite of passage for any serious DIYer or mechanic. It is a task that rewards the patient and punishes the hurried.

By investing in a high-quality set of cobalt left-hand bits and taking the time to properly center-punch your workpiece, you can handle almost any snapped fastener with confidence. Remember to use plenty of cutting oil, keep your speeds low, and let the tool do the work.

Don’t let a broken bolt ruin your day. Take a deep breath, grab your extraction kit, and show that hardware who’s boss. You’ve got the knowledge and the tools—now go get that project back on track!

Jim Boslice
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