Symptoms
- Bolt or screw sheared off flush with or below the surface
- No head or shank left to grab with a wrench
- Threaded hole now has a metal stub stuck inside
- Project can't proceed until it's removed
Likely causes
- CommonBolt was rusted or seized before breakingCorrosion made it stick, and force applied to free it sheared the metal instead.
- CommonOvertightening beyond the fastener's rated torqueExcess force exceeds the metal's strength and snaps the shank.
- PossibleMetal fatigue from vibration or ageRepeated stress over years weakens a fastener until it fails.
- Less commonWrong grade or brittle fastener used originallyA hardened but brittle bolt can snap rather than bend under load.
What to do
- Center-punch the broken bolt's flat surface so a drill bit won't wander.
- Drill a small pilot hole straight down the center of the broken fastener.
- Use a spiral screw extractor (turns counterclockwise and bites in as it's twisted) or an extractor socket sized to the hole.
- Turn the extractor slowly and steadily; stop if it feels like it's slipping rather than gripping.
- If extraction fails, drill the stub out completely and re-tap the hole to the same or next thread size.
- Clean out all metal shavings before installing a new fastener.
- Consider using anti-seize compound on the new fastener's threads to prevent this from happening again.
Tools and parts
- Center punch
- Drill and bits
- Screw/bolt extractor set
- Tap and die set (if re-tapping)
Replacement bolt or thread insertSearch: “helicoil thread repair insert kit”
When to call a pro
If the broken fastener is in an engine block, expensive casting, or hard-to-reach spot, a machine shop can extract it or weld on a removal nut without ruining the part.
Safety
Use caution
Wear eye protection when drilling metal; broken extractor bits are hardened and brittle and can shatter if forced.