Symptoms
- Allen key or hex wrench spins in the bolt head
- Corners of the bolt head look worn or rounded
- Socket slips off under torque
- Bolt won't turn despite visible contact
Likely causes
- CommonWorn or wrong-size Allen key used repeatedlyA slightly undersized key rounds the internal flats over time.
- CommonMetric/imperial size mismatchUsing a close-but-wrong size (like 8mm on a 5/16 head) rounds it quickly.
- PossibleCorrosion filling the recessRust reduces contact area so the tool doesn't seat fully.
- Less commonBolt was overtightened originallyExcess torque can deform the head shape before removal is even attempted.
What to do
- Clean the recess or head flats with a wire brush to remove dirt, paint, or rust.
- Try the next size up of Allen key or socket, tapped in firmly to bite the remaining metal.
- For rounded external hex heads, use a 12-point or locking pliers-style wrench that clamps onto the corners instead of fitting over them.
- Add a rubber band or valve-grinding compound between the tool and bolt for extra grip on a stripped internal recess.
- If it still won't turn, use an extractor socket made for rounded fasteners, which bites in as torque is applied.
- As a last resort, cut a slot into the bolt head with a rotary tool and use a flathead screwdriver.
- Replace the fastener afterward rather than reusing a rounded one.
Tools and parts
- Allen key/hex socket set
- Locking pliers
- Extractor socket set
- Wire brush
Replacement hex or Allen boltSearch: “socket head cap screw or hex bolt replacement”
When to call a pro
On furniture, bikes, or equipment where the rounded bolt is in a hard-to-access spot, a repair shop with specialized extraction tools can save the surrounding part from damage.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.