How to Remove a Stripped or Broken Bolt: 5 Pro Workshop Methods

    To remove a stripped or broken bolt without damaging the internal female thread, apply one of these 5 workshop methods based on bolt condition: cut a flat drive slot with a rotary tool if the head protrudes, drive in a Torx bit on rounded Allen sockets, center-drill with a 5% cobalt bit and use a reverse spiral screw extractor (easy-out), MIG-weld a hex nut onto flush-sheared shanks, or apply thermal shock with a torch and penetrating fluid to break seized rust bonds.
 

    Few things cause more frustration in the workshop than the dry snap of a high-tensile bolt shearing off flush with the engine block, or a driver bit spinning freely inside a stripped fastener head. The common knee-jerk reaction is to strike it recklessly with a cold chisel or force cheap HSS drill bits that wander off-center, permanently chewing through the parent casting.
 

    At Bulonera Alvear, we regularly see valuable castings and housings ruined by poor technique and substandard tooling. Below, we break down each professional extraction procedure step-by-step alongside the exact drill-to-extractor sizing chart to save your internal threads.
 

Decision Matrix: Which Method to Use by Problem Type

Method 1: Stripped Drive Socket (The Torx Trick & Slot Cutting)

    When the internal hex drive of an Allen bolt or the cross recess of a Phillips screw has completely rounded out, but the head remains intact:
 

   
• 
        The Torx Bit Trick (for stripped Hex/Allen bolts): Select a Torx star bit slightly larger than the rounded opening (for example, a T30 or T40 bit into a stripped 5 mm Allen socket). Drive it in firmly with a ball-peen hammer. The sharp hardened splines will bite fresh grooves into the steel, allowing you to break it loose with a hand ratchet without slipping.
     

   
• 
        Cutting a Straight Drive Slot (for round or button heads): Using a thin 1 mm cutoff wheel on an angle grinder or rotary tool (like a Dremel), grind a clean, deep slot straight across the center of the head. Then use a heavy-duty flathead screwdriver with a striking cap or a manual hand impact driver to shock it loose.
     

 

    To learn more about drive profiles and torque capacities, explore our comprehensive guide on fastener head styles and drive types.
 

Method 2: Reverse Spiral Screw Extractor ("Easy-Out")

    This is the industry standard when a bolt snaps off flush with the surface or down inside a threaded blind hole. It features left-hand tapered spiral flutes that wedge deeper into the bolt core and unscrew it as you turn counter-clockwise:
 

   
1. 
         Flatten the Fractured Surface: If the break left jagged, uneven ridges, dress it flat with a file or burr so the drill bit will not skate sideways.
      

   
2. 
         Center Punch (Non-Negotiable Step): Using a hardened steel center punch and a firm hammer blow, dimple the exact geometric dead-center of the broken bolt. Drilling off-center will permanently bore into the female casting threads.
      

   
3. 
         Pilot Drilling with a Cobalt Bit: Medium and high-tensile fasteners (Class 8.8, 10.9, or SAE Grade 8) will instantly burn and dull standard HSS drill bits. Use a 5% Cobalt Alloy bit (HSS-Co / M35) at low RPM with a few drops of cutting fluid or tapping oil.
      

   
4. 
         Engaging the Extractor: Insert the correct extractor into the pilot hole using a manual T-handle tap wrench. Turn it firmly counter-clockwise by hand; as the tapered spirals bite into the steel, the broken shank will break free and back out.
      

 

Sizing Chart: Bolt Size vs. Pilot Drill Bit vs. Extractor Number

Method 3: Welding a Nut (The Master Workshop Technique)

    This is by far the most reliable method for medium to large bolts (M8 / 5/16" and up) broken flush inside cast iron engine blocks or heavy structural steel plates:
 

   
• Place a standard steel hex nut with an inner diameter matching or slightly smaller than the broken bolt stub directly over the fracture.

   
• Using a MIG welder or stick electrode (such as E7018 or specialized stainless/cast electrode), puddle weld down through the center hole, directly fusing the nut to the core of the broken bolt.

   
• The intense heat does 80% of the extraction work: Massive localized thermal expansion crushes the rust crust along the thread interface and instantly vaporizes hardened threadlocking compounds.

   
• Let it cool down for 2 to 3 minutes until the dull cherry red glow subsides, douse it with penetrating oil, and gently unscrew the nut with a box wrench or standard six-point socket.

 

Methods 4 & 5: Thermal Expansion & Thread Chasing

   
• 
        Method 4: Torch Heating & Impact Shock: In humid subtropical climates, iron oxide expands between thread flanks, seizing fasteners solid. Heat the surrounding housing with a portable butane or MAPP gas torch so the female cavity expands faster than the bolt shank. Apply aerosol penetrant and strike the center punch sharply with a hammer to break structural shear tensions.
     

   
• 
        Method 5: Thread Tap Cleanup (The Critical Finish): Once the sheared bolt is out, never screw in a replacement fastener immediately. Run a matching thread tap or thread chaser coated with light machine oil through the hole to wipe out metal filings and oxidized scale. Review thread pitches in our guide on mechanical thread standards and pitch verification.