Disconnect the trimmer from power, wait until the head is still, then open the housing and remove leftover line. Load the diameter of monofilament the head is designed for. Wind a rewindable spool tightly in the marked direction without crossing strands, route the ends through the eyelets, reassemble, and trim the tails to the guard’s cut-off length so the head can feed.
String trimmer line is a consumable. It frays, snaps, and shortens until the head no longer reaches the grass. Changing it is a mechanical job, not a brand-specific ritual: make the tool inert, identify how that particular head holds cutting line, load the right monofilament, and put the housing back together so the line can advance. Designs differ, so treat markings on the spool and the owner’s manual as the final authority whenever they conflict with a general walkthrough.
Make the Trimmer Safe Before You Open the Head
A cutting head can start if the power source is still connected and a trigger, switch, or interlock is bumped. Do the disconnect first, then wait until the head is fully still. Do not reach for leftover line, packed grass, or a cover tab while anything is spinning.
Disconnect gas, corded, and battery models
The line-change itself is similar on gas, corded, and battery string trimmers. The power-disconnect step is not.
- On a gas model, release the trigger, switch the engine off, and keep your hands away from the throttle. Disconnect the spark-plug lead so a residual charge or an accidental pull of the starter cannot fire the engine while the head is open. Set the lead aside so it cannot spring back onto the plug.
- On a corded electric model, release the trigger and unplug the power cord from the outlet. Unplugging at the wall is more reliable than relying on a switch or an extension-cord connection that someone else might restore.
- On a battery model, release the trigger and remove the battery pack. Do not leave the pack in the tool with the switch merely off.
Follow any additional lockout or storage steps printed on the tool or in its manual. Do not bypass guards, interlocks, or kill switches to make the head easier to reach.
Wait for the head, clear the area, and protect your eyes
After shutdown, wait until the cutting head has stopped completely. A still-spinning disc can cut fingers, fling fragments, or yank leftover line. Work away from bystanders, pets, and traffic. Brush loose clippings off the guard so you can see the housing, eyelets, and any release tabs. Leave the guard in place for the later test; you only need access to the head itself.
Eye protection is warranted. Old monofilament can snap, housing edges can shed plastic, and the first spin after reassembly can throw debris. Close-fitting gloves help when you pull packed grass or handle a spring that may jump out of the drum. You do not need specialized shop gear for a typical home trimmer, but bare hands and no eye protection are a poor combination for this job.
Identify the Head Type and Whether You Can Rewind It
Replacement method follows the head, not the power source. Most consumer heads fall into three families: bump-feed, automatic-feed, and fixed-line. Some of those heads accept bulk line on a rewindable spool, some take a drop-in pre-wound spool, and some lock short pre-cut pieces into slots. Forcing a rewind on a head that was never meant to be rewound wastes time and can crack the housing.
Bump-feed, auto-feed, and fixed-line heads
A bump-feed head is the most common home design. Line is stored on a spool inside a drum. Tapping the knob on the ground while the head is spinning releases a short length of line through the eyelets. You will usually see a rounded bump button on the bottom, one or two exit holes on the rim, and a cover that unscrews or unclips.
An auto-feed head also stores line on an internal spool, but it advances line by centrifugal force, a clutch, or another automatic mechanism instead of a deliberate bump. The housing may look similar to a bump head. The clue is in the manual or on the cover: if the instructions talk about automatic feed and there is no bump knob, treat it as auto-feed when you test later.
A fixed-line head does not use a rewindable spool. It holds short, pre-cut pieces of cutting line in slots, clamps, or holes around the rim. When a piece wears down, you pull the stub and lock in a new length. There is no winding direction and no dual-channel spool to manage.
Rewind the spool, drop in a pre-wound spool, or use short pieces
If the cover comes off and you can lift a spool with molded channels, arrows, or wording such as “wind this way,” the head is usually user-rewindable with bulk line. If the spool is a sealed cassette or the manual specifies a replacement cartridge, install a pre-wound spool rather than fighting the old one. If the head has rim slots and no inner drum, use the short pieces it was designed for.
Some rewindable heads also accept a matching pre-wound spool as a convenience. That is a valid shortcut when the original spool is cracked, the spring is missing, or you do not want to wind line in the field. It is not a reason to force a cassette into a head that only takes bulk line, or the reverse.
Single-line versus dual-line
Look at the housing and the spool together. One eyelet and one channel usually mean a single-line setup. Two opposite eyelets and a spool divided into two halves mean dual-line. Dual-line spools often have a through-hole or two separate anchor slots so each strand stays in its own half. Count the exit holes before you cut line. Loading two strands into a single-line drum, or one strand into a dual-line drum that expects both sides filled, is a common reason the head will not feed.
If the housing is sealed, cracked, or has no obvious release tab, knob, or split line, stop guessing. Open designs vary enough that a sealed or damaged head belongs in the model’s instructions, not in a generic pry-apart attempt.
Choose Line That Fits the Head
Wrong diameter is the most common reason a freshly loaded head jams, refuses to bump, or keeps snapping. Shape and color are secondary. Fit comes first.
Find the diameter and the strand count
Look for a size marking on the head, the spool, a sticker under the cover, or the owner’s manual. That marking is the diameter the eyelets, channels, and feed mechanism were built around. Use that size. Do not assume every consumer string trimmer takes the same monofilament, and do not “size up” for toughness if the head is marked for a thinner line. Oversize line binds in the eyelets and can stall the motor or overheat a small electric unit. Undersize line may feed too freely, weld itself into a lump, or break at the eyelet.
Confirm whether the head needs one strand or two. Dual-line heads need two working tails of the same diameter. Mixing leftover scraps of different sizes in one spool is a reliable way to make only one side advance.
Bulk line or a pre-wound replacement spool
Bulk line is a coil or spool of monofilament that you cut and wind yourself. It is the usual choice when the original spool is intact and clearly marked with a winding direction. A pre-wound replacement spool is a ready-made cassette or loaded reel that drops into the drum. Choose the pre-wound route when the head was designed for it, when the old spool is damaged, or when you want to skip winding.
Round, twisted, and multi-sided profiles all cut grass if they fit. A fancier shape does not excuse the wrong diameter. If you like a particular profile, buy it in the size the head lists.
When the old line is gone and the head is unmarked
If no size is printed on the parts you have, match any remaining stub still in an eyelet, or compare a short sample to line that is still labeled. If nothing remains, consult the manufacturer’s documentation for that model rather than inventing a diameter. Hardware-store “universal” line is only universal when it happens to match the head you own.
Open the Housing and Clear the Old Line
Work on a bench or a clean patch of ground so small parts cannot vanish in the grass. Typical bump-feed and auto-feed covers come apart in one of a few ways: squeeze opposing tabs and lift the cap, unscrew a large knob or the bump button, or twist the cover to a marked unlock position and pull it off. Turn the head so you can see the seam. Do not pry with a screwdriver against thin plastic unless the manual shows a pry point; cracked covers are a replace-the-head problem.
Lift the spool and note how the spring sits
After the cover is off, lift the spool straight out. Many heads hide a coil spring under the spool or inside the bump knob. That spring can jump. Cup a hand over the assembly as you separate the parts, then set the spring in a cup or on the cover so you remember which way it faces. The spring’s job is to keep tension so the spool can click and feed. If you reverse it or leave it out, the head may freewheel, refuse to bump, or rattle itself apart.
Photograph or mentally note the order of washer, spring, spool, and cap if the stack looks more complex than a single reel. Treat any sequence you see as typical for that head, not as a universal exploded diagram.
Clear leftover line and packed debris
Pull remaining monofilament off the spool. Cut welded lumps rather than forcing them through an eyelet. Pick packed grass, mud, and broken line out of the drum, the eyelets, and the cover. Grit in the drum is enough to keep a freshly wound spool from turning. Check that each eyelet is still a smooth hole and not a razor-edged groove. A worn eyelet will chew new line as soon as you start cutting.
If a tab, clip, or inner washer came free, keep it with the spring. Feed mechanisms depend on those small parts staying in the same order. If a piece is missing or the housing is cracked through, rewind will not restore safe operation.
Cut, Insert, and Wind the New Line
This section is for rewindable spools loaded with bulk line. If you have a pre-wound spool, skip to seating that cassette in the drum. If you have a fixed-line head, skip to locking short pieces into the rim slots. Do not unwind a sealed cassette to “do it properly.”
Size the new pieces from the old line, the spool, or the model
There is no single length that fits every consumer head. Use one of these guides, in this order: the length printed in the model’s instructions, the amount that filled the old spool without climbing over the rim, or a piece that matches the leftover line you just removed. Cut less than you think you need if you are unsure. An overfilled spool binds; you can always add a bit more on the next change. Dual-line heads usually take two equal lengths, or one long piece folded in half, depending on whether the spool has a through-hole.
Straighten a short working section of new monofilament so it will thread. Cold line can be stiff; warming it in your hands for a moment makes it easier to seat, but do not heat it with a flame or a heat gun.
Thread single-line and dual-line spools
On a single-line spool, push one end into the anchor hole or slot until it catches, leaving a short tail inside the hub if the design calls for it. On a dual-line spool with a through-hole, push one long piece through until the mid-point sits in the hole, so each half of the spool gets an equal run. On a dual-line spool with two separate anchors, seat each piece in its own slot. Keep the two halves in their own channels from the first wrap. Crossing at the hub is as bad as crossing later.
Wind in the marked direction, firmly and evenly
The arrows or wording on the spool are the authority for winding direction. Wind the way they point. Winding against the arrows is a frequent cause of failed bump or auto feed: the head spins, the line tries to tighten instead of release, and nothing comes out of the eyelets.
Wind under light, even tension so the coils lie flat and do not spring loose. Do not yank so hard that you stretch or flatten the monofilament. Stay inside each channel. On a dual-line spool, wind both sides together or one side and then the other, but never let one strand climb the divider and land on its neighbor. Leave short working tails long enough to reach through the eyelets after the spool is back in the drum. Most spools have notches on the rim to park those tails so the reel does not unwind in your hands.
Stop before the line stands above the rim or packs the drum so tightly that the spool cannot turn. Loose coils that later expand under heat will bind just as surely as an overfill. There is no universal wrap count; fill the channels, not the entire cavity.
Seat a pre-wound spool or lock fixed pieces
A pre-wound spool drops into the drum in a specific orientation. Align any tabs or D-shaped hubs, pull the factory tails out to the eyelets, and do not strip the line off to rewind it unless the cassette is clearly a standard open reel. On a fixed-line head, insert each pre-cut piece through the slot or hole until the lock, bend, or retainer captures it. Equal lengths on opposite sides keep the head balanced. Tug each piece to confirm it is captured before you restore power.
Reassemble, Set Line Length, and Check the Feed
Hold the wound spool so the parked tails cannot slither back into the channels. Set the spring back in the same orientation you found it. Lower the spool into the drum so it sits flat and can rotate. If the spool has a hub that must engage a shaft or a set of teeth, turn it gently until it drops on.
Route the tails and close the cover
Unhook each tail from its parking notch and push it out through the matching eyelet. Both tails on a dual-line head must exit; a tail trapped under the cover will pinch, melt, or keep the cap from seating. Fit the cover so any alignment marks or tabs match the housing. Press or twist until every latch clicks, or thread the knob until it is snug. The cover should sit flush. If it rocks, a tail is pinched, the spring is cocked, or the spool is not fully down.
Shorten the line to the guard
Most guards include a small cut-off blade on the rim. With the guard installed, pull or bump the line until the tails reach that blade, then spin the head briefly later so the blade trims the excess. If you are still assembling by hand, cut the tails to about the length the guard allows so they will not slap the shield or flail past it on the first start. Do not leave long streamers “for extra reach.” Extra length stresses the head, can hit the user or the shield, and does not cut a wider path once the cut-off blade does its job.
Confirm feed before regular cutting
Restore the spark-plug lead, plug, or battery only after the cover is fully latched and the guard is on. Move to a clear area away from people, glass, and loose gravel. Start the trimmer and let the head come up to speed. For a bump-feed head, tap the knob on bare soil or turf and watch for a short advance of line. For an auto-feed head, run it briefly and see whether line is present at both eyelets and the head is not laboring. Stop immediately if you hear grinding, smell hot plastic, or see the head wobble.
Do not test with the guard removed. The cut-off blade, the shield, and your eyesight are part of the same system. A brief test is enough to prove the line exits and advances; it will not tell you a cutting width, an advance length per bump, or how long the new line will last.
Fix Common Line-Change Problems
When a freshly loaded head will not cut, the cause is usually mechanical and visible once you open it again. Work through the likely faults before you replace the entire trimmer.
Line will not feed
If nothing comes out when you bump or when an auto-feed head should advance, open the housing and look for winding against the arrows, dual strands that crossed, an overfilled rim, or coils that sprang loose and expanded against the drum. Any of those conditions lock the spool. Rewind in the marked winding direction, keep each half in its channel, and leave a little free space under the cover. Also check that the spring is present and seated and that the tails actually pass through the eyelets instead of sitting under the cap.
Wrong diameter shows up here too. Line that is too thick will not slide through the eyelets. Line that is far too thin may slip without engaging the feed mechanism. Go back to the marking on the head or spool.
Line keeps breaking or welds into a lump
Repeated snaps at the eyelet often mean a worn, grooved eyelet acting like a cutter, old brittle monofilament, or a habit of hitting fences, concrete, and tree bark. Replace a chewed eyelet or the whole head if the hole is no longer smooth. Fresh line that is several seasons old can shatter even when the diameter is correct; store spare line out of direct sun and do not expect oxidized coils to behave like new ones.
A solid lump of fused line inside the drum is heat-weld. Loose windings slap, friction rises, and the monofilament melts together so it cannot advance. Cut the lump out, clean the drum, and rewind firmly enough that the coils cannot chafe. Overfilling and the wrong diameter make welding more likely because the spool cannot turn freely.
Winding mistake versus worn parts
A winding mistake is reversible: reverse direction, uncross the strands, reduce the fill, or switch to the marked diameter and the head should work. A damaged eyelet, a weak or missing spring, a spool that will not turn on its hub, or a cracked housing will fail again as soon as you close the cover. If the drum is oval, the cover will not latch, or the bump button is broken off, stop rewinding. Replace the spool, the spring, or the entire cutting head with parts specified for that model.
Fixed-line heads have a shorter list of faults. If a piece will not lock, the slot is worn or you are using the wrong length or diameter of pre-cut line. If the head vibrates hard after a change, the pieces are uneven or one side is empty.
When to replace the spool or the whole head
Rewind when the plastic is intact, the spring still has tension, and the eyelets are smooth. Replace the spool if it is cracked, the divider between dual-line halves is broken, or the hub is worn so the reel wobbles. Replace the head when the housing is split, the eyelets are ruined, the cover will not stay latched, or you cannot identify a safe way to open a sealed unit. Continuing to force line into a damaged head is how fragments leave the tool at speed.
Once the correct monofilament is in a sound head, wound in the marked direction, and trimmed to the guard, the trimmer should cut again. The next change will be faster because you already know whether that head is bump-feed, auto-feed, or fixed-line, and whether it wants bulk line, a pre-wound spool, or short locked pieces.
Frequently asked questions
Can I mix leftover line with a new coil?
Only if it is the same diameter and you can keep the strands from crossing. Mixing sizes in one dual-line spool is a common feed failure. When in doubt, strip the old monofilament and start clean.
Does line shape matter more than diameter?
No. Round, twisted, and multi-sided cutting line can all work. The head’s marked diameter and the single- versus dual-line layout matter more than profile. Choose a shape you like only after the size is correct.
Why did the line melt after a few minutes of use?
Loose or overfilled windings can rub until the monofilament heat-welds into a lump. Wrong diameter and packed debris in the drum cause the same binding. Open the head, clear the drum, and rewind firmly in the marked direction without filling past the rim.
Do I need the owner’s manual if the spool has arrows?
Arrows are enough for winding direction on a typical open spool. You still want the manual when the housing will not open in an obvious way, the head is unmarked for diameter, or the tool uses a sealed pre-wound cassette. Those details are model-specific and should not be guessed.
