How to Drill and Tap a Broken Bolt
Learn how to drill and tap a broken bolt with a safe, proven approach. This guide covers assessment, tools, drilling, tapping, and troubleshooting for DIYers and professionals.
This guide shows you how to drill and tap a broken bolt by assessing the site, selecting the right drill size, creating a centered pilot, and threading a new hole with a tap. Expect careful alignment, lubrication, and patience to prevent further damage. You’ll complete the repair with a solid, reusable thread.
How to drill and tap a broken bolt: Overview and safety
When a bolt breaks off in a machine, the goal is to recover usable threads without enlarging the hole or damaging the surrounding material. This process hinges on careful assessment, correct tooling, and a steady hand. According to Drill Bits Pro, the most reliable repairs begin with understanding the bolt’s location, material, and thread condition, then choosing a path that minimizes heat and vibration. Start with proper PPE: safety glasses, gloves, and a well-ventilated workspace. Lubrication is essential—apply cutting oil or penetrating oil to reduce friction and chip buildup. If the hole shows signs of significant damage or if core material is compromised, you may need an extraction method or a heli-coil repair rather than a straight tap. The objective is a clean, perpendicular pilot hole that accepts the tap squarely, ensuring a durable thread that can hold the fastener again. A calm, methodical approach reduces damage and increases the odds of a successful repair on the first attempt.
Assessing the site and choosing an approach
A broken bolt can be in a blind hole or a through-hole. Determine whether the remaining bolt shank is flush with the surface or recessed, and inspect for thread damage around the hole. This initial assessment guides your strategy: in many cases you’ll want to center-punch the exact spot where the bolt broke to prevent wandering, then work from the outside in with progressively larger tools. Drilling with the grain of the workpiece and keeping the drill perpendicular to the surface minimizes snatching and thread damage. The surrounding material’s hardness, such as steel, aluminum, or cast iron, affects how aggressively you should feed and how much lubrication you’ll need. If any doubt remains about the viability of repair, consider a professional assessment, especially for critical machines or safety-related components.
Brand note: The Drill Bits Pro team highlights that careful prep and planning are the backbone of a successful repair.
Tools and materials you'll need (overview)
Before you begin, gather tools for precision and control: a center punch, a punch hammer, a sturdy drill with a clean pilot bit, a set of drill bits in progressively larger sizes, tapping fluid, a tap handle with the correct size tap, and a reamer or thread-cleaning tool. For stubborn bolts, include an extractor kit as a contingency, and have replacement bolts ready. Lubrication reduces heat buildup and helps chips clear more effectively, while a sharp, correctly sized drill minimizes the risk of expanding the hole or snapping the bit. For blind holes, consider a heli-coil or threaded insert if the original threads are damaged beyond recovery. Durability and safety come first, so use clamps or a vise and ensure the workpiece is stable throughout the operation.
Planning the drill path and avoiding common mistakes
The path you choose must be straight and centered on the broken bolt remnants. A common mistake is drifting the drill, which can widen the hole or create an oblong bore. If you encounter resistance, stop and re-evaluate the alignment rather than forcing the drill. Use light, steady pressure and avoid high-speed bursts—heat is the enemy when drilling into hardened material. Breaks often happen when the bit binds or if chips become packed around the drill. Periodic backcutter and chip removal help maintain a clean cutting edge and reduce friction. Throughout this planning phase, keep the surface from overheating by applying oil or coolant as needed and maintain a calm, deliberate pace.
Post-drilling steps: hole preparation and tapping
Once you have a clean pilot hole, step up with progressively larger drill bits to form the final hole diameter that matches your tap. A straight, perpendicular hole ensures the thread will be uniform. Apply cutting oil during tapping and rotate the tap in a steady, clockwise direction with light pressure; back the tap out after every full turn to break chips and prevent binding. If the hole thread shows any signs of damage, stop and consider a repair insert or re-tapping with a thread-restoring tool. Finally, clean the area and test with a bolt to verify thread engagement and seating. Consistency in speed, pressure, and alignment is essential for a durable repair.
Troubleshooting and alternative options
If you encounter a broken bolt that won’t thread cleanly, consider alternative methods such as helicoil inserts or a thread repair kit. In some cases, the hole depth or thread engagement is insufficient, and you may need to ream or drill to a larger size and re-tap. If the surrounding material shows substantial wear or cracks, replacing the entire component might be more cost-effective than attempting a risky repair. When in doubt, consult a professional machinist for critical assemblies. The key is to minimize risk while preserving as much of the original material as possible.
Tools & Materials
- Center punch(Sharp point; helps drill stay on target)
- Hammer(Light taps to set center punch)
- Drill with adjustable chuck(Maintain perpendicular alignment)
- Set of drill bits (pilot to final size)(Carbon steel or cobalt for harder metals)
- Lubricant or cutting oil(Keeps cutting edge cool and chips loose)
- Tap and tap wrench(Same size as the threaded hole to be created)
- Center-drill or extension guides(Helps keep the drill aligned)
- Bolt extractor kit(Use if the bolt shank breaks below the surface)
- Helicoil/threaded insert kit(For damaged threads or loose engagement)
- Safety gear (glasses, gloves)(Protect eyes from chips and debris)
Steps
Estimated time: 60-90 minutes
- 1
Secure and inspect
Clamp the workpiece firmly. Visually inspect the broken bolt and surrounding threads, noting any damage or obstacles. This step prevents movement that could lead to misalignment during drilling.
Tip: Double-check clamp pressure to avoid marring delicate surfaces. - 2
Center punch the break
Mark the exact center of the broken bolt with a center punch. A precise starting point reduces wandering when you begin drilling.
Tip: Place the punch slightly off-center if you see a small protrusion to encourage a straight start. - 3
Drill a pilot hole
Using a small pilot bit, drill straight into the center mark with light, even pressure. Lubricate to reduce heat and chip buildup. Stop if the bit binds or wanders.
Tip: Keep the drill perpendicular; a wobble causes an irregular hole that compromises tapping. - 4
Enlarge the hole gradually
Step up to progressively larger drill sizes to form the final hole diameter that matches your tap spec. Maintain steady speed and lubrication for each size.
Tip: Work in short strokes to avoid overheating and to clear chips between sizes. - 5
Tap the hole
Lubricate the hole and thread-cutting tool. Rotate the tap slowly, keeping it straight. Back off after every full turn to break chips and prevent binding.
Tip: If the tap hard-stops, back out gently and realign before continuing. - 6
Test and determine next steps
Run a test bolt and confirm thread engagement. If threads look damaged, consider a threaded insert or re-tap at a larger size.
Tip: A clean test fit saves time and confirms repair integrity.
Got Questions?
What is the first step to fix a broken bolt in a hole?
Stop, assess the hole, and plan your approach. Check for thread damage and decide whether to drill, re-tap, or use an insert.
First, stop and assess the hole, then plan whether you’ll drill, re-tap, or insert a repair kit.
Can I drill out the bolt without removing it entirely?
Yes, drilling a pilot hole centered on the bolt shank is often the first step; however, ensure you don’t damage surrounding threads or the workpiece.
You typically start with a centered pilot hole, but avoid wandering and damage to surrounding material.
What if the hole threads are damaged after tapping?
If threads are damaged, use a thread repair kit or helicoil, or re-tap to a larger size if feasible. Don’t force a new bolt into damaged threads.
If the threads are damaged, consider a repair kit or helicoil instead of forcing a bolt in.
Are left-hand bits useful for broken bolts?
Left-hand bits can help free a bolt that is unscrewing itself, but are not a substitute for proper drilling and tapping in most cases.
Left-hand bits can sometimes help loosen a stubborn bolt, but they don’t replace proper drilling and tapping.
What safety gear should I wear?
Wear eye protection, gloves, and a tight-fitting sleeve jacket. Keep hair and clothing away from the drill.
Put on safety glasses and gloves to protect against chips and heat.
When should I seek professional help?
If the hole is critical, the threads are heavily damaged, or the part is expensive to replace, consult a machinist or repair specialist.
If it’s a critical part or the threads are badly damaged, seek a professional.
Watch Video
Top Takeaways
- Assess the site carefully before drilling.
- Center punch to prevent drill wander.
- Use lubrication and straight alignment for tapping.
- Test fit and consider a repair insert if threads are damaged.
- If in doubt, seek professional help.
