String trimmer line keeps breaking when it is the wrong diameter, brittle from age or heat, wound incorrectly, or repeatedly striking concrete, fences, rocks, and other hard surfaces. Start by checking the line size and condition, then inspect the spool, eyelets, and head. If those parts are sound, adjust your trimming technique and loading method before suspecting the motor.
1. Check that the line is the correct size
String trimmers are designed for a specific line diameter range. Line that is too thick can overload the head or motor, while line that is too thin may snap quickly in heavier grass and weeds.
Check the trimmer's owner's manual, label, or manufacturer's instructions for the approved diameter. Do not choose line only by appearance. The correct size depends on the trimmer's power, head design, and intended use.
Also check the line shape. Round, square, twisted, and serrated lines can behave differently, and some heads are intended for particular line types. If the line was recently changed, reinstall the recommended type and size before troubleshooting anything else.
2. Replace old, brittle, or heat-damaged line
Trimmer line becomes less flexible as it ages. Line stored in direct sunlight, a hot shed, or a vehicle can become brittle and break with light contact.
Look for line that is:
- Dry, cracked, chalky, or visibly brittle
- Flattened or kinked from being tightly stored
- Discolored from heat or sunlight
- Difficult to bend without snapping
Remove the old line and load fresh line from a sealed package. Store unused line in a cool, shaded place. Some manufacturers recommend soaking certain nylon lines in water to restore flexibility, but follow the line maker's instructions rather than assuming all line should be soaked.
3. Inspect the spool and winding pattern
Poor winding can cause the line to cross over itself, bind in the spool, or feed unevenly. The sudden release of a stuck section can then snap the line at the eyelet or inside the head.
With the trimmer switched off and disconnected from its power source:
- Remove the spool or bump-feed cap according to the manual.
- Clean out grass, dirt, and melted plastic.
- Check for cracks, warped plastic, or a damaged center hub.
- Wind the line in the direction marked on the spool.
- Keep each wrap neat and next to the previous wrap.
- Do not overfill the spool.
- Leave the specified amount of free line at each end.
If the spool is cracked, badly worn, or no longer holds the line securely, replace it with a compatible spool rather than trying to repair it.
4. Check the eyelets and trimmer head
The eyelets are the openings where the line exits the head. A sharp edge, burr, missing guide, or worn eyelet can cut the line every time it rotates.
Turn off the trimmer and inspect both exit points. Run a fingertip carefully around the plastic or metal opening, watching for sharp damage. Do not force your finger into a narrow opening; use a cloth or cotton swab if needed.
Replace the eyelet, guide, cap, or complete head if the manufacturer supplies it as a service part. A damaged head can continue breaking new line, so changing line alone may not solve the problem.
5. Avoid wrapping the line around the head
Line can break at the head if it is routed incorrectly or trapped under the spool. Compare the installation with the diagram in the owner's manual. The line should exit through the intended guides and move freely when the spool is turned by hand.
For bump-feed heads, make sure the cap, spring, spool, and retaining parts are assembled in the correct order. A missing or incorrectly positioned spring can prevent proper feeding and create excess tension on the line.
Never modify the head to accept more line than specified. Overfilling can cause binding, overheating, and repeated breakage.
6. Change your trimming technique
Even correctly installed line will break quickly when it repeatedly hits hard objects. Use the outer portion of the line's cutting arc and move gradually through tall or dense growth instead of forcing the head into it.
Common causes of impact breakage include:
- Striking concrete, asphalt, brick, or stone
- Hitting chain-link fencing or metal edging
- Cutting directly against tree trunks or masonry
- Allowing the line to contact curbs and walls
- Moving too quickly through thick, woody weeds
- Using a light-duty trimmer for brush it was not designed to cut
Keep the head parallel to the ground for ordinary grass unless the manual specifies another position. Near obstacles, slow down and leave a small margin that can be finished with a more suitable tool. For heavy brush, use only a blade or brush-cutting attachment approved for that trimmer and installed according to the manufacturer's instructions.
7. Check for a head that is spinning too fast or dragging
If the line breaks during normal grass trimming, inspect how the head operates. A head that drags against the ground, is packed with wet grass, or has a damaged bearing or cover can put unusual stress on the line.
Switch off the trimmer, disconnect the battery or unplug it, and clear packed debris. The head should rotate freely by hand when the trimmer is safely disabled. Do not operate the tool with a cracked guard, loose head, missing fastener, or damaged shaft.
If the head vibrates heavily, makes unusual noise, or becomes excessively hot, stop using the trimmer. Those symptoms can indicate a damaged head, shaft, gearbox, or motor and may require a qualified repair technician.
8. Reload the line correctly
A reliable reload depends on using the exact line length and routing shown in the manual. General steps are:
- Remove the power source before opening the head.
- Clean the head and spool.
- Cut line to the manufacturer's recommended length.
- Wind it tightly and evenly in the marked direction.
- Keep the line under the spool's retaining tabs if present.
- Feed both ends through the correct eyelets.
- Reassemble the head fully before reconnecting power.
- Confirm that the line extends evenly and is not trapped.
Avoid pulling excessive line from the head to compensate for poor feeding. If the automatic or bump-feed mechanism does not release line correctly, inspect the cap and spool for wear or replace the feed components.
When should you replace the head or trimmer?
Replace the spool or head when it has cracked, warped, developed sharp exit points, or repeatedly damages correctly sized fresh line. A replacement head is also reasonable when the feed mechanism is worn and replacement parts are unavailable or cost nearly as much as a compatible head.
Consider replacing the trimmer when the head is sound, the correct line is installed, and the line still breaks during light work—especially if the tool vibrates, overheats, loses power, or has a damaged shaft or gearbox. Stop using it and follow the manufacturer's service guidance for electrical or motor faults. Do not bypass guards or attempt repairs to energized wiring.
Quick diagnosis
- Line snaps immediately after loading: Check the diameter, winding direction, line length, and spool assembly.
- Line breaks at one specific exit point: Inspect that eyelet for a burr, crack, or missing guide.
- Line breaks only near concrete or fences: Change technique and keep the cutting arc away from hard surfaces.
- Line breaks after storage: Replace brittle or heat-damaged line.
- Line feeds poorly and then snaps: Clean the head and inspect the spool, spring, cap, and retaining tabs.
- Fresh line breaks during light grass trimming: Check for a damaged head, excessive vibration, or a mechanical fault.
