Symptoms
- Head visibly wobbles while spinning
- Uneven cutting height across a swipe
- Rattling noise from the head area
- Head appears loose when wiggled by hand while off
Likely causes
- CommonHead not fully threaded on or tightenedA head that's only partially threaded on will spin off-center.
- CommonWorn or damaged drive shaft splinesRounded-off splines let the head sit slightly loose even when tightened.
- PossibleBent trimmer head housingImpact from a rock or hard surface can bend the plastic housing out of round.
- PossibleCross-threaded installationInstalling the head at an angle can prevent it from seating flush.
- Less commonWorn gearbox bearing (curved shaft models)Internal bearing wear at the gearbox can introduce wobble unrelated to the head itself.
What to do
- Turn off the trimmer and remove the head to inspect the drive shaft splines for wear or rounding.
- Clean any dirt or grass fibers from the shaft and inside the head mount.
- Reinstall the head, making sure it's threaded straight and tightened fully by hand plus a quarter turn with the shaft locked.
- Spin the head slowly by hand once installed to feel for any wobble before starting the engine.
- Inspect the head housing itself for visible warping or cracks and replace if damaged.
- If wobble persists with a good head and clean shaft, check the gearbox bearing on curved-shaft models for play.
- Test at idle first, then gradually increase throttle to confirm the wobble is gone.
Tools and parts
- Shaft lock pin
- Clean rag
Trimmer headSearch: “trimmer head replacement”
Gearbox assemblySearch: “curved shaft trimmer gearbox replacement”
When to call a pro
If the drive shaft splines are worn or the gearbox bearing has play, a small-engine shop can replace the affected part.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.