Steel and carbide blades do not fail gradually and politely most of the time. They fail because they were already cracked, chipped, or bent and nobody caught it before the next start-up. On a machine spinning a blade at 8,000-10,000 rpm at the tip, that is not a minor maintenance oversight, it is the difference between a clean cut and a piece of metal leaving the head at speed. The checks below take two minutes before each job and they are the ones that matter.
Safety note: Always stop the engine, let the blade come to a complete stop, and disconnect the spark plug lead before inspecting or handling any blade. Wear cut-resistant gloves; even a “worn out” blade edge can slice a finger.
1. Any crack, however small
This is the one sign with zero judgement calls attached. If you see a crack anywhere in the blade, at the base of a tooth, radiating from the centre bore, along a stress line, the blade is scrap. Do not grind it smooth, do not “keep an eye on it,” do not finish the row you are on. Bin it immediately.
Cracks form from metal fatigue: repeated flexing from vibration, a stone strike, or a mounting bolt that was over- or under-torqued. Once a crack starts, the remaining metal carries more stress with every rotation, and it propagates faster than it looks. A blade that separates at full rpm becomes a piece of steel travelling at roughly 90-110 km/h at the tip radius on a typical 255 mm (10 in) 3-tooth blade. That is well beyond the reach of any guard, which is why manufacturers set a 15 m clearance for bystanders in the first place.
Inspect under good light, and run a thumbnail or a cloth along stress points, hairline cracks can be hard to see but easy to feel as a snag.
2. Missing or chipped teeth
A tooth that has broken off, or chipped back significantly from its original profile, does two things: it stops cutting properly on that segment, and it throws the blade out of rotational balance. An unbalanced blade vibrates harder, which accelerates wear everywhere else, on the gearbox, the shaft bearing, and your hands and wrists over a full day’s work.
- One or two small nicks on a tooth edge: usually filable, keep going.
- A tooth missing more than roughly a third of its length, or a chip that reaches the tooth’s base: replace the blade.
- Any asymmetry where one tooth is visibly shorter or a different shape than its opposite number: replace, do not try to “match” it by grinding the others down.
Chipped teeth are most often caused by hitting a fence wire, a buried stone, or stump material the blade was never designed to cut. If you are chewing through mostly woody stumps, the correct tool is a dedicated brush blade with more aggressive tooth geometry, not a repeatedly-repaired scrub blade.
3. Bent or warped disc
Lay the blade on a flat surface, a workbench or a sheet of glass, and check it sits flush all the way round. Any blade that rocks, has a visible wave, or shows daylight under one section at the edge is warped and needs replacing.
A bent blade cuts unevenly, pulls to one side in use, and, more importantly, is now spinning with its mass off the original plane. That produces the same vibration and stress problems as a chipped tooth, just distributed differently. Warping usually comes from a hard sideways strike (a fence post, a rock at an angle) rather than straight-on contact, so check after any jarring hit even if the edge looks fine.
4. Worn below the tooth depth
Every tooth has a working depth, the gullet between teeth that lets cut material clear the blade. As you sharpen a blade repeatedly over its life, that depth shrinks. Once the gullet is nearly gone, and the tooth is little more than a shallow notch, the blade cannot clear material properly and cutting performance drops even when the edge itself is sharp.
| Wear stage | What you see | Action |
|---|---|---|
| Light wear | Edge slightly dulled, gullet depth mostly intact | Sharpen, keep using |
| Moderate wear | 2-3 sharpenings in, gullet visibly shallower | Sharpen carefully, inspect closely each time |
| Heavy wear | Gullet nearly flat, tooth is a shallow bump | Retire the blade |
| Any crack/chip/warp | — | Retire immediately, regardless of edge condition |
As a rule of thumb, a steel blade tolerates a handful of sharpenings, perhaps 4-6, before the tooth geometry is compromised enough to retire it. Carbide-tipped blades hold an edge much longer but cannot usually be resharpened by hand at all; when the tips wear or chip, the blade is replaced rather than serviced.
5. Enlarged or oval centre bore
The centre bore is what keys the blade to the drive shaft and takes the entire rotational load. Over time, especially if the blade has ever been fitted loosely or the retaining nut has backed off in use, the bore can wear oval rather than staying round.
Check this every time you remove and refit a blade:
- Clean the bore and the drive flange of debris.
- Fit the blade and feel for any play or looseness against the flange, it should sit snug with no rocking.
- Fit the retaining washer and nut, and torque to the spec in your manual, do not just “nip it up by feel” on this one.
- Spin the blade by hand before starting the engine and check it runs true.
An enlarged bore lets the blade shift slightly under load, which shows up as vibration that seems to come from nowhere, and in the worst case allows the blade to walk off-centre or detach entirely. If the bore has visible ovality or the blade does not sit snug against the flange, replace it, this is not a wear point worth pushing.
6. Corrosion and pitting
Surface rust after a wet storage season is cosmetic and comes off with a wire brush. What matters is pitting, small pockmarks eaten into the metal surface, especially near the tooth roots or around the centre bore. Pitting is metal loss, and it creates the same stress-concentration points that starts a crack. A blade with heavy pitting along an edge or around the bore should be retired even if it has not visibly cracked yet, because it is the most likely place for one to start next.
Store blades dry, ideally with a light coat of oil if they will sit unused over winter, and they will not need this check nearly as often.
Typical lifespan expectations
There is no fixed hour count because it depends entirely on ground conditions, but as a working guide:
- Steel 3-tooth or 4-tooth blade, mixed bramble and light scrub, average garden or smallholding use: one to three seasons before retirement, sharpened several times along the way.
- Carbide-tipped blade, similar use: significantly longer edge life, often several seasons, but replaced rather than sharpened once tips chip or wear.
- Heavy daily commercial use, verges, contract clearing, stony ground: a blade may need retiring within weeks to a few months, and it is normal to carry spares.
- Any blade that has taken a hard strike (fence wire, buried metal, a rock at speed): inspect immediately regardless of how many hours are on it. Age in hours means nothing next to one bad impact.
Quick pre-start checklist
Run through this before every session, it takes less time than reading this paragraph:
- Any crack anywhere on the blade → stop, replace.
- Teeth all present, none chipped back to the base → sharpen or replace as needed.
- Blade lies flat, no warp or wave → replace if bent.
- Tooth gullets still have real depth → replace if worn flat.
- Centre bore round, blade seats snug, nut torqued to spec → replace if oval or loose.
- No pitting at tooth roots or bore → replace if heavily corroded.
When to call a dealer
Most of this is a five-minute job with a spare blade in the shed. Call a dealer or small-engine mechanic if you are seeing repeated bore wear or vibration even with a known-good blade fitted, that points to a worn drive shaft, gearbox, or output bearing rather than the blade itself, and running it further risks damaging the head assembly. Likewise, if you are unsure whether a mark is a crack or just surface scratching, treat it as a crack and get a second opinion before the next start-up. A blade costs little next to a trip to A&E.