The engine note is the giveaway here: if it revs freely and cleanly with no load at all, whatever’s wrong is downstream of the crankshaft, not in the fuel or ignition system. Work from the clutch outward toward the head, since that’s also the order from cheapest to most expensive fix.
Safety first. Every check below involves the clutch, drive shaft, or gearbox at some point spinning or being spun by hand. Disconnect the spark plug lead before opening any cover, and never test a clutch or shaft with the blade or head still fitted — remove the cutting attachment first.
Work through the drivetrain in this order
| Symptom | Most likely cause | Check first |
|---|---|---|
| Head is completely dead, no drive at all, any rpm | Sheared drive square, disconnected shaft, or badly stripped gearbox | Drive square, shaft engagement |
| Engine revs, slight drag felt through the shaft, but head stays still | Worn clutch shoes or broken clutch spring | Clutch assembly |
| Drive comes and goes, or feels “spongy” when you twist the shaft by hand | Flexible drive shaft worn, kinked, or snapped inside its housing | Flexible shaft (curved-shaft machines) |
| Head spins slowly or roughly, grinding noise from the gearbox end | Stripped or worn gearbox gears | Gearbox |
| Head was fine, then stopped suddenly after hitting a stone or fence wire | Seized head or sheared drive square from shock load | Head bearing, drive square |
| Head turns a little by hand but engine can’t drive it under load | Slipping clutch (worn shoes) or a gearbox on the verge of stripping | Clutch first, then gearbox |
1. Worn clutch shoes or a broken spring
This is by far the most common cause and the cheapest to fix. Nearly every handheld brush cutter uses a centrifugal clutch: at idle the shoes sit retracted off the drum, and above roughly 3,000-4,000 rpm they fling outward under centrifugal force, grip the drum’s inner surface, and drive the gearbox. Worn friction material, a snapped or badly weakened spring, or a scored drum all break that grip.
How to check:
- Disconnect the spark plug lead and remove the blade or head.
- Remove the clutch cover (usually 2-3 screws, sometimes tucked behind the starter housing).
- With the lead still off, pull the starter cord slowly and watch the drum. On a healthy clutch you’ll feel and see the shoes engage as the cord speeds up; a worn clutch will spin the shoes without ever gripping.
- Inspect the shoes for rounded, glazed, or thinned friction material, and check the spring isn’t stretched, cracked, or sitting crooked.
The fix: replace the clutch as a complete assembly rather than sourcing an individual spring — it’s a low-cost part, typically in the €10-25 range, and mixing an old spring with new shoes (or vice versa) risks uneven engagement. Fitting is usually a reverse-thread nut holding the clutch to the crankshaft; check your manual before forcing it loose, and hold the flywheel with a strap wrench or the correct locking tool rather than jamming the crankshaft some other way.
Clutch shoes engage under real centrifugal force at working rpm. Never run the engine with the cover off to watch it “in action” at anything above idle — inspect it stationary, spun only by hand or slow cord-pulls with the plug lead disconnected.
2. Failed flexible drive shaft (curved-shaft machines)
Curved-shaft strimmers and light brush cutters use a flexible drive shaft — a tightly wound steel cable running inside the shaft tube — rather than a solid rod. These shafts wear, kink, or snap over years of use, especially if they’ve run dry of grease or been bent sharply during storage or transport.
Signs it’s the flexible shaft:
- The clutch engages fine (you can feel or hear it grip when you pull the starter with the head removed) but nothing reaches the gearbox.
- Twisting the shaft by hand at the head end feels loose, springy, or disconnected compared with turning it at the engine end.
- A grinding or clicking noise from inside the shaft tube that changes with engine rpm.
- The shaft has visibly separated or frayed if you can see the ends after removing the tube.
How to check:
- Disconnect the shaft tube from the gearbox housing (usually a couple of bolts or a spline collar).
- Pull the inner flexible shaft partway out and inspect for kinks, rust, dry patches with no grease, or strands that have unwound.
- Try rotating it by hand from each end — a sound shaft transmits rotation firmly along its whole length; a broken one will spin freely partway along with no resistance beyond the break point.
The fix: a snapped or badly worn flexible shaft needs replacing as a unit — they aren’t practically repairable. Grease the new shaft generously with the lubricant specified in your manual before fitting (often a lithium-based grease, applied along the full length, not just the ends), since running dry is what kills these shafts prematurely. Expect a flexible shaft replacement in the roughly €15-35 range for the part on most consumer machines, plus your time.
3. Stripped or worn gearbox gears
The right-angle gearbox at the head end takes the full torque of cutting, and its gears can strip teeth from sudden shock loads (hitting a fence post, a buried stone, or heavy wire) or wear gradually from age and neglected lubrication. A gearbox on the way out often gives warning — grinding, roughness, or intermittent drive — before it fails completely.
Signs it’s the gearbox:
- Grinding, whining, or a rattling feel transmitted through the shaft as the head tries and fails to turn smoothly.
- The output shaft feels notchy, gritty, or has noticeable play when you rotate it by hand with the blade removed and engine off.
- Grease that’s black, gritty with metal flecks, or clearly overdue — check your manual’s interval, commonly every 25-50 hours or once a season.
- The fault appeared suddenly right after a hard impact with a solid obstacle.
The fix: open the grease fill screw and check condition and level first — low or contaminated grease accelerates gear wear and is worth ruling out before assuming the gears themselves are stripped. If teeth are visibly rounded, chipped, or missing, the gearbox needs a rebuild or full replacement; this is specialist work needing gear pullers, the correct grease, and often exact-match replacement gears by model, so it’s genuinely a workshop job rather than a driveway fix.
4. Sheared drive square or disconnected coupling
The drive square is the small square-section stub — either on the gearbox output shaft or at the base of the drive shaft — that the head or blade adaptor locks onto. A sudden, hard stop (blade jamming solid against a stone, wire wrapping tight around the shaft) can shear this square off cleanly, or on split-shaft machines, simply knock the coupling out of engagement.
Signs:
- The failure was instant, usually right after the blade hit something solid and stalled hard.
- With the head removed, the output shaft spins but the square end looks rounded, twisted, or visibly broken off compared with a fresh part.
- On split-shaft machines, check the coupling collar is fully engaged and the locking pin or lever is properly seated — a shaft that isn’t pushed in and locked all the way will spin the upper section with no drive reaching the lower one at all.
The fix: a sheared drive square usually means replacing the output shaft or the short adaptor stub it’s part of, depending on design — a relatively inexpensive part on most gearboxes, but the labour of stripping the gearbox to reach it makes this borderline DIY territory depending on your confidence with hand tools. A disconnected split-shaft coupling is a two-minute fix: reseat it fully and confirm the lock is engaged before restarting.
5. Seized head
Less about the drivetrain and more about the attachment itself: if the clutch, shaft, and gearbox all check out but the head still won’t turn, the head’s own bearing or bump-feed spool may have seized from corrosion, packed grass, or a bent shaft stub.
Signs:
- Everything upstream (clutch, shaft, gearbox output) spins freely and correctly when tested in isolation.
- The head itself is stiff or immovable by hand once detached, or shows rust and no play at all in what should be a free-spinning bearing.
- Grass, cord, or wire tightly wound around the spindle behind the head, jamming it solid.
The fix: strip the head, clear any packed debris from around the spindle, and check the bearing spins freely once clean. A seized bearing on a bump-feed or blade adaptor is usually a low-cost replacement part; corrosion from storing the machine wet or outdoors is the most common cause, so drying and light-oiling the head before long storage periods prevents most repeats.
Rough repair cost guide
| Fault | Typical part cost (EUR) | DIY-friendly? |
|---|---|---|
| Clutch shoes/spring (replace as assembly) | €10-25 | Yes, straightforward |
| Flexible drive shaft | €15-35 | Yes, with correct grease |
| Sheared drive square/adaptor | €10-30, plus gearbox strip labour | Moderate |
| Seized head bearing | €5-15 | Yes, straightforward |
| Gearbox rebuild (bearings, gears) | Often close to or exceeding a replacement gearbox unit | No — workshop job |
| Full gearbox replacement | Varies widely by brand/model | Moderate to difficult depending on shaft type |
Quick checklist
- Confirm the engine revs freely with no load — if so, the fault is downstream of the crankshaft.
- Remove the head, pull the starter slowly with the plug lead off, and watch whether the clutch shoes engage the drum.
- On curved-shaft machines, check the flexible shaft transmits rotation firmly along its full length.
- Check the gearbox output shaft for smooth rotation, correct grease level, and no metallic grit.
- Inspect the drive square for shearing and confirm any split-shaft coupling is fully locked.
- Strip and check the head itself last, once everything upstream has tested sound.
When to call a dealer
Clutch replacement, greasing or replacing a flexible shaft, reseating a split-shaft coupling, and clearing a seized head are all reasonable jobs for anyone comfortable with basic hand tools and a manual open beside them. A stripped gearbox, a sheared drive square buried inside the gearbox housing, or any fault you can’t pin down after working through the checks above is worth taking to a small-engine mechanic — gearbox rebuilds need pullers and exact-match parts, and getting it wrong on a reassembled gearbox risks turning a repairable fault into a scrapped machine. If a gearbox quote comes back close to the cost of a new gearbox unit or a large fraction of a replacement machine, it’s worth weighing the repair against buying new before committing.