Open a spool and find two or three coils fused into a solid clump, and the line usually won’t feed at all once you get there — the bump mechanism just grinds against a lump of melted nylon. It’s a heat fault, and once you know where the heat is coming from it’s straightforward to stop it happening again.
Why the line fuses in the first place
Nylon trimmer line has a fairly low softening point — well under the temperature a spool head can reach in hard use. Trimmer line generates friction heat in two places: against the eyelet or line guide as it feeds out, and against itself where adjacent coils sit pressed together under spool tension. Get either hot enough for long enough and the outer surface softens just enough to bond to the coil next to it. As the head cools down, that softened nylon sets solid, and you’re left with a welded lump instead of line that pays out freely.
A few things push the temperature up:
- Hard, sustained cutting — dense scrub, thick weeds, or long uninterrupted running sessions build up more friction heat than light trimming ever will.
- Constant bump-feed advancing — every bump momentarily increases friction at the eyelet as fresh line is pulled through, and repeated bumping in quick succession doesn’t give the head time to cool between feeds.
- Cheap or off-spec line — low-grade nylon often has a lower softening point and less consistent diameter, so it runs hotter for the same cutting load than a quality line rated for the head.
- Line wound too tightly or unevenly — overlapping coils crossing over each other create hot spots where they rub, rather than each wrap sitting cleanly against the last.
- A head that’s already running warm — a gearbox or bump-head mechanism that’s picked up heat from the engine or from friction in its own moving parts adds to whatever the line itself is generating.
None of this is a spool defect. It’s a symptom of how the line is being run, wound, or stored, which is good news because all three are within your control.
Never try to pull a fused clump apart with the engine running or the head still attached to a spinning shaft. Remove the spool, let it cool fully, and work on it by hand with the machine off and the plug lead disconnected.
Prevention: the habits that actually stop it
1. Use quality line rated for your head
Not all trimmer line is equal. Reputable brands (Oregon and the major brush cutter manufacturers all sell their own) spec their line’s diameter and material to match typical head temperatures and eyelet designs. Cheap bulk line bought purely on price often runs a fraction hotter for the same job and is more prone to welding, especially in dense or wet growth where friction is already higher.
| Line type | Typical use | Heat tolerance |
|---|---|---|
| Round, standard nylon | Light-medium trimming, mixed grass | Moderate — fine for most garden work |
| Twisted/square profile | Thicker weeds, better cut quality | Slightly better, cuts more aggressively so less dwell time per pass |
| Co-polymer or heavy-duty | Dense scrub, semi-woody growth | Higher softening point, worth it for hard use |
| Very cheap unbranded bulk line | Any | Lowest tolerance — most likely to fuse under load |
If you’re routinely clearing heavy growth, stepping up to a heavier-duty or co-polymer line is a cheap fix (typically only a couple of euros more per roll) that pays for itself in fewer melted spools and fewer stop-start jobs.
2. Avoid excessive bumping
Bump-feed heads are designed to advance line as needed, not on every few seconds of cutting out of habit. Bumping the head against the ground repeatedly, especially in quick succession, keeps re-introducing friction at the eyelet before the head has had a chance to cool. Let line wear down naturally and bump only when you can hear or feel the cut quality drop off, rather than pre-emptively feeding fresh line every few passes.
3. Wind evenly and keep layers separate
How the line goes onto the spool matters as much as how it comes off:
- Wind in the direction marked on the spool (most are marked with an arrow) — winding backwards causes the line to unwind against the head’s rotation.
- Keep each wrap snug against the previous one, without overlapping or crossing over.
- If your spool has two channels, keep the two lengths of line separate the whole way through — don’t let them cross between channels, as crossed line rubs and generates heat internally even before it reaches the eyelet.
- Don’t overfill the spool. Packing in more line than the spool is designed for increases internal pressure between coils, which increases the contact heat between them.
4. Give the head time to cool on hard jobs
On a long session in thick material, pause every 15-20 minutes to let the head and line cool, especially in hot weather. This is a small habit change but it matters more than it sounds — a head that never gets a chance to shed heat between short bursts accumulates it wrap by wrap.
Soaking line before use
Nylon line that’s been sitting for weeks or months, especially in a dry or heated space, loses some flexibility and moisture content. Brittle line is more prone to snapping under load, and snapping under load often triggers more bumping to feed fresh line, which brings you straight back to the heat problem above.
How to soak it:
- Coil the length of line loosely (or leave it on a fresh spool if pre-wound).
- Submerge it in a bucket of water for 10-15 minutes.
- Dry the outer surface before loading it, so you’re not winding wet line into a spool that will trap moisture against the mechanism.
This restores flexibility and makes the line easier to wind evenly, but it does nothing for line that’s already welded or scorched — that line needs replacing, not rehydrating.
Storage that prevents the problem before it starts
- Keep spare line indoors, or in a shed/garage with stable temperature — not on a sunny windowsill or in a hot roof space where UV and heat dry it out over a season.
- Store it in its original packaging or a sealed container where practical, to keep humidity roughly constant.
- Don’t leave a loaded spool in direct summer sun in the boot of a car or on a trailer between jobs — the plastic spool itself can warm enough to soften line sitting inside it.
- Rotate stock: use older rolls before newer ones rather than leaving one roll at the back of the shed for a couple of seasons while working through others.
Quick checklist
- If line is already fused, remove the spool, let it cool, and re-spool with fresh line rather than fighting the clump apart.
- Step up to a heavier-duty or co-polymer line if you regularly cut dense growth.
- Bump the feed only when cut quality actually drops, not as a routine habit.
- Wind evenly, keep dual channels separate, and don’t overfill the spool.
- Pause on long hot jobs to let the head cool.
- Soak brittle old line for 10-15 minutes before winding it onto a spool.
- Store spare line indoors, away from heat and direct sun.
When to call a dealer
This is a consumables and technique issue, not a mechanical fault, so there’s nothing here that needs a workshop visit. The one exception is if line keeps welding even after you’ve switched to quality line, wound it correctly, and avoided excessive bumping — that pattern can point to a head or eyelet running hotter than it should, which is worth having a small-engine mechanic check for a damaged eyelet, a dragging bump mechanism, or a gearbox running warmer than normal.