A bolt or screw turns and turns but never gets tight, or it comes out with bright aluminium caught in its thread. The female thread in the casting has been pulled out. It is common on aluminium crankcases, covers and cylinder heads, because steel bolts are much stronger than the soft metal around them. Causes are overtightening, a bolt that was cross threaded, the wrong pitch bolt, or a bolt run in with an impact gun.
Is it safe to ride?
It depends on what the bolt holds. A loose side cover screw can wait a short time if the cover does not leak. Do not ride if the stripped hole holds a brake part, an axle, an engine mount, a cylinder head or anything in the steering. A drain plug that will not tighten can drop out and empty the engine.
Quick checks first
- What does the bolt hold? If it is a safety part, stop riding.
- Is the bolt the right one for that hole? A bolt from another hole may be the wrong pitch or length.
- Look at the bolt thread. Bright metal in it means the hole is damaged. Damaged bolt threads mean the bolt is bad too.
- Is the part leaking oil around the bolt?
Diagnose it step by step
1Is the bolt the correct size, pitch and length for that hole?
- Not sure: check it: Identify a thread standard and pitch. A wrong bolt is the most common cause.
- Yes: go to step 2.
2Does a new, correct bolt tighten by hand to full depth?
- Yes, but it was overtightened: the old bolt was stretched or damaged. Fit the new one and tighten to torque: Use a torque wrench.
- No: go to step 3.
3Is only the first part of the thread damaged, with good thread further down?
- Yes: the thread may clean up with a tap of the right size: Repair a stripped thread.
- No, most of the thread is gone: go to step 4.
4Is it a lightly loaded hole (cover, panel, bracket)?
- Yes: fit a thread insert of the original size: Repair a stripped thread.
- No, it is a head stud, engine mount, axle or brake mounting: workshop job. Ask for a thread insert or a welded repair, and for the bolt to be torqued to the manual value.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Overtightened, often with no torque wrench | Bolt stretched, thread pulled | Stripped thread repair |
| Wrong bolt (pitch or length) | Pitch gauge, bolt bottoms out | Identify the thread |
| Cross threaded on fitting | Bolt went in crooked, stiff after a turn | Stripped thread repair |
| Run in with an impact gun | Several holes damaged on the same cover | Use an impact wrench safely |
| Corrosion between steel and aluminium | White powder, bolt seized then pulled thread out | Stuck fastener removal |