Disconnect the trimmer’s power source, open the head, and wind compatible cutting line evenly onto the spool in the direction marked on the spool or housing. Route the ends through the eyelets, close the head without overfilling, then confirm the line extends when you bump or activate the feed. Head designs differ, so treat the arrows, line-size stamp, and owner’s manual as the authority for your tool.
What this job involves
Winding trimmer line is a maintenance task, not a repair mystery. You are replacing the nylon cutting string that a string trimmer spins out of the trimmer head. A successful rewind looks ordinary when you return to work: the cover sits flush, the free ends exit the eyelets at a usable length, and a bump or the head’s feed system pays out more line instead of locking, snapping, or going silent.
Most consumer bump-feed heads, and many similar single-line or dual-line designs, share the same logic. You isolate power, open the housing, load line that matches the stamped diameter, wind in the marked direction, keep the wraps even, and put the spring and cover back in the right order. Auto-feed heads, fixed-line heads, and pre-wound cartridges follow that safety logic too, but they do not share identical parts or winding paths.
This guide stays model-agnostic on purpose. It will not replace the arrows on your spool, the size stamp on the head, or the owner’s manual. Stop following a generic sequence and switch to those sources when the housing will not open the way a typical bump-feed cap does, when there is no rewindable spool, when the head uses short inserted pieces instead of a coil, or when a matching pre-wound spool is the part the tool was designed to accept.
What stays the same across common heads is the reader goal: load compatible line so the head can feed and cut without fighting itself. What changes is the path the line takes, how the cap releases, and whether you are winding loose line or swapping a cartridge. Treat those differences as design categories, not as a ranking of which head is better.
Make the trimmer safe before you open the head
The cutting head can start if the trigger, lockout, or a residual electrical path still has power. Isolate the tool first. Remove the battery from a cordless trimmer and keep it away from the handle. Unplug a corded trimmer at the outlet, not only at an intermediate connector. For a gas trimmer, many manuals tell owners to disconnect the spark-plug lead so a pull on the starter cannot fire the engine while your hands are on the head. Follow the isolation steps printed for your machine rather than inventing a wait time or a fuel-drain ritual that the manual does not ask for.
Do not bypass interlocks, tape a trigger, or “just be careful” with a live battery still clicked in. Unexpected startup is the hazard that turns a simple rewind into an injury.
Put on eye protection before you pop a cap or pull old line. Spring-loaded bump knobs, cracked nylon, and short offcuts can jump when the housing opens. Face protection is worth adding if the old line is brittle or the head is packed with debris. Gloves help with sharp eyelet edges and with line that has mushroomed from heat. This is practical protection, not a substitute for reading the tool’s own safety labels.
Stabilize the trimmer so it cannot roll or hang from the shaft while you work. A bench, a clear patch of ground, or a vise that holds the shaft without crushing it is enough. Lay out a tray or a light-colored cloth for the bump knob, retaining spring, washers, and any clips. Those small parts disappear into grass as soon as the spool comes out, and their order matters later. Work in enough light to see arrows, size stamps, and the divider on a dual-line spool before you start pulling line.
Choose line that actually fits the head
Compatible line diameter is usually stamped on the trimmer head, molded on the spool, printed on a label near the guard, or listed in the product manual. There is no single correct size for every consumer string trimmer. If the stamp and the manual disagree in wording, treat the documentation for that exact head as the authority and do not guess from a bulk spool that “looks close.”
Line that is too thick may not seat in the eyelets, may bind inside the housing, or may load the motor and gearbox harder than the head was built to take. Line that is too thin can whip, break at the eyelet, or fail to trigger a bump-feed or auto-feed mechanism that expects a stiffer strand. Those are practical fit problems, not a promise about cutting speed.
Shape is a handling choice, not a performance ranking. Round line is usually the easiest to wind because it lies in even layers and does not have edges that catch on themselves. Twisted, serrated, or multi-sided line can feel stiffer in the hand and may want more care to keep wraps from climbing the spool wall. Describe those differences only as they affect winding. Do not assume a shape lasts longer, cuts faster, or uses less fuel.
Length is not universal either. Buy more line than you think you need so you can discard fused or kinked sections, but wind only what that spool can hold below its rim or fill mark. A matching pre-wound replacement spool is often the better choice when the head is designed as a cartridge system, when the original spool is cracked, or when you would rather not reconstruct a dual-line divider by hand. Installing the correct cartridge is still a fit job: the hub, drive dogs, and eyelet alignment have to match that head.
Open the head and remove the spool without losing parts
Typical bump-feed caps release in a few common patterns. Some twist off after you press tabs. Some unclip when you squeeze opposite sides. Some unscrew after you hold the spool still. Look at the housing before you apply force. Arrows, “open” marks, and tab locations tell you the intended motion. Forcing a twist on a clip-on cap, or prying a threaded cover with a screwdriver, is how plastic ears break.
Before the spool comes free, notice what is still attached to it. Line may still be routed through the eyelets. A bump knob may be part of the cover or a separate button on the spool. A retaining spring often sits under the spool or inside the bump assembly. Washers or a small plate may sit on the shaft. Mentally photograph that stack, or set the parts down in the order they came off. Reassembly fails more often from a spring left on the bench than from a bad wind.
If the head has no rewindable spool, stop. Fixed-line heads use short cut pieces that lock into slots or holes. Some auto-feed designs hide extra pawls, felt pads, or one-way mechanisms that are not meant to be pried apart like a simple bump-feed cup. A head that only accepts a sealed cartridge should be opened only as far as the manual shows for swapping that cartridge. If nothing looks like a consumer bump-feed spool, do not force the housing open to invent a winding path.
Clean loose grass and packed dirt from the housing while it is open. Grit in the spring seat or under the spool can keep the bump mechanism from moving even after a perfect rewind. Do not soak electrical parts or a gas engine in solvent to clean the head; wipe the plastic and metal you can see.
Load the line and wind in the marked direction
Find the entry point on the spool before you start wrapping. Many single-line spools have a hole, slot, or anchor notch where one end of the line locks. Dual-line spools often have a center hole or two opposing anchors so you can fold a length in half or load two separate pieces. Seat the line in that lock so the first wrap cannot spin free when you begin winding.
Wind direction is not a universal clockwise rule. Use the arrows or “wind this way” marks on the spool, the inner housing, or both. Those marks exist because the bump-feed or auto-feed mechanism pays line out against a specific wrap direction. Winding the opposite way is one of the most common reasons a freshly loaded head will not feed. If one mark is on the spool and another is on the housing, they should agree once the spool sits in its normal orientation. If they do not, stop and check the manual rather than averaging the two.
On a dual-line or divided spool, keep each half in its own channel or on its own side of the divider. Load so both sides build at the same pace. If you wind one side fat and the other sparse, the cover may tilt, one eyelet may starve, and the lines can climb over the divider and cross. Some dual systems want a single folded length; others want two cut pieces. Follow the path molded into that spool instead of copying a neighbor’s head.
Leave enough free end on each line to pass through the eyelets and still extend outside the housing after the cover goes on. The exact leftover length varies by head. A practical target is enough line to reach from the spool, through the eyelet, and a short working distance past the rim—long enough to cut later against the guard knife if your tool has one, short enough that the ends do not tangle around the shaft while you close the cap. Do not treat any measurement in feet or meters as correct for every spool.
If you are installing a pre-wound spool instead of hand-winding, skip the wrapping step and align the factory-wound cartridge so its exit notches match the housing eyelets. You are still responsible for direction and seating: a cartridge dropped in backward can refuse to feed even though the line looks neatly wound.
Keep the wind even and stop before the spool is overfull
The line should sit snug, not drum-tight and not floppy. A good layer looks like hose on a reel: each wrap beside the last, climbing in even rows, staying below the spool rim or below any fill mark molded on that spool. Hold light tension with one hand and rotate the spool with the other so you are laying line, not yanking it into a fused lump.
Overfill shows up before you ever pull the trigger. The cover will not seat. The bump knob binds. The outer wraps sit above the rim and get pinched by the housing. Dual lines start to climb the divider. If you see any of those signs, unwind a portion and stop at a lower, even level. An overfilled or crossed wind can keep the cover from seating and can stop the line from paying out.
On a dual spool, watch the divider on every few wraps. If one strand starts to jump the wall, back up and relay that section. Do not mash the jumped wrap down and keep going. Crossed dual lines lock against each other inside the head and are a frequent cause of “new line, no feed.”
Old line that is welded, flattened, or permanently coiled into a set should come off the spool entirely. Heat from storage in a hot shed, from a previous overfilled run, or from friction in the eyelets can fuse nylon to itself. Prefer replacing damaged line over improvised solvents or heat. Scraping fused nylon with a knife also risks gouging the spool, which then catches fresh wraps. If the plastic spool itself is cracked, melted, or missing the divider, replace the spool rather than winding onto a damaged core.
Reassemble the head and confirm the line feeds
Route the free ends through the eyelets, or through the matching notches in the cap if your design uses both, so each line can exit the housing without crossing. Then rebuild the stack in the order you removed it. On many bump-feed heads that order is retaining spring, spool, then bump knob or cover. Some designs put the spring in the cover. Typical is not guaranteed: if a washer sat on the shaft, it goes back where it came from. The spool must sit down on its drive features so it cannot spin independently of the head.
Close the cover until the tabs click or the threads bottom evenly. A cap that only catches on one side is not closed. If it will not go on, do not hammer it. Recheck fill height, line routing, and whether the spring is cocked out of its seat. Trim the external ends only after the head is together, and only to a length that clears the debris shield. If the tool has a cutoff knife on the guard, that knife is meant to set length after a careful, shielded test spin—not as an excuse to run the head without the guard.
Reinstall the debris shield if you removed it. Eye and face protection still matter because cut line and debris can fly off when the head is running. Restore power only after the housing is fully assembled: battery in, cord plugged in, or spark-plug lead reconnected according to the manual.
Test feed with the guard in place and your body clear of the cutting path. For a bump-feed head, a brief run and a tap of the bump knob on bare ground or a firm surface should extend line if the wind and spring are correct. For an auto-feed head, use the feed method that design expects, still with the shield on. You are checking that line pays out and that the cover stays put. You are not measuring cutting width or claiming the tool now cuts like new.
If the cap will not close, reopen and look for overfill, a crossed dual wind, or a spring out of place. If the line will not extend, reopen and verify wind direction against the arrows, confirm the ends actually pass through the eyelets, and confirm the spool can rotate the small amount a bump or auto-feed system needs. If the head rattles or the bump button feels dead, the retaining spring may be missing or inverted.
Adapt the method to other common head types
Installing a matching pre-wound spool differs from hand-winding mainly in where the work happens. The factory wind should already be even and directional. Your job is identification and seating: the cartridge must be the one made for that head, the exits must line up with the eyelets, and the drive hub must engage. You still isolate power and you still confirm feed with the guard on. You do not unwind a good cartridge “to check it” unless the manual says to finish a partial wrap.
Fixed-line heads are a different category. They use short inserted pieces, not a wound coil. You cut or buy lengths that fit the slots, lock each piece as the head design requires, and you do not treat the housing as a rewindable bump-feed spool. Forcing those heads open to add a coil can break the locking features and still will not create a feed system the tool never had.
Auto-feed heads add extra caution because more internal parts manage the payout. Extra springs, one-way clutches, or sensing arms can sit under the spool. Open only as far as needed to replace line or the intended cartridge. Do not bend tabs to “make it feed more.” If the mechanism is a sealed module, replacement of that module may be the intended service path rather than a field rewind.
Sometimes the head itself is worn out and not worth another rewind. Visual signs are enough to decide: a cracked housing, rounded or egg-shaped eyelets that saw through line, a missing or broken retaining spring, melted plastic around the hub, or a bump knob that no longer moves. New line cannot restore a head that no longer guides or releases that line. Replace the damaged part or the complete head with the type specified for the tool.
If the line still will not feed or keeps breaking
After a careful rewind, most feed failures trace back to a short list of winding mistakes rather than a mysterious defect. Wrong direction relative to the arrows leaves the mechanism trying to tighten the coil instead of paying it out. Overfill pinches the outer wraps. Crossed dual lines lock against the divider. Line diameter that does not match the stamp will not move cleanly through the eyelets. A missing spring or a spool that is not down on its drive dogs can make a bump-feed head feel dead.
Line sometimes welds to itself after heat or storage. A tightly wound spool left in a hot vehicle, or a head that ran with the line already rubbing the housing, can fuse layers. The symptom is a head that fed once and then stopped as if empty. The fix is to discard the fused section and rewind fresh line evenly, not to cook the spool or soak it in household chemicals.
Repeated breaks at the eyelet often mean the eyelet is worn sharp, the line is too thin for that head, or the external length is so long that it whips and fatigues at the metal or plastic edge. Inspect the eyelet with a fingertip (power still isolated). A groove you can feel is a reason to replace the eyelet insert or the head. Fresh line through a razor-edged hole will keep failing.
Replace the spool or the whole trimmer head instead of winding again when the plastic is cracked, the divider is gone, the spring cannot be seated, or the bump path is so worn that the button no longer lifts the spool. Repeating the same rewind on damaged parts wastes line and can throw fragments when the tool runs. This is troubleshooting for common winding faults, not a shop repair manual. Do not treat adhesives, heat guns, or improvised bushings as standard fixes.
Limits, damage risks, and when to get model-specific help
Oversized line and an overfilled spool are not just inconvenient. They can stall the head, overheat a motor, stress a gearbox, or crack the cover the next time you bump it. Those outcomes are reasons to stay inside the stamped diameter and below the rim, not reasons to quote a load rating this article does not have.
The debris shield stays on after you refill the line. It deflects cut grass and broken nylon away from you and, on many tools, carries the knife that trims excess length. Running a string trimmer without that guard to “see if it feeds” turns a checkout into an avoidable hazard. Keep bystanders out of the cutting path during any test.
Go back to the owner’s manual, the size stamp, and the winding diagram for your exact head when the housing does not match a simple bump-feed pattern, when an auto-feed module will not reassemble, or when the tool shows electrical, fuel, or structural problems that a spool of line cannot address. A qualified service shop is the right next step if the shaft, clutch, or motor is damaged, if you cannot isolate power confidently, or if repeated correct-looking rewinds still fail and the head is not an obvious visual write-off.
Trimmer heads are not all built the same, so arrows, size stamps, and the owner’s manual override generic winding advice. A clean, even wind in the marked direction, with compatible line and a complete spring-and-cover stack, is what lets a common bump-feed or similar head feed and cut. Everything beyond that is model-specific, and it belongs in the documentation that came with the tool.
