How to Put Trimmer Line on a String Trimmer

To put trimmer line on a typical bump-feed head, disconnect power, open the head, and wind fresh line onto the spool in the marked direction—or feed a measured length through a quick-load slot. Seat the tails in the eyelets, reassemble the cap so the line can move, then test the feed with the head pointed away from people. Match line diameter and head type first; fixed-line, auto-feed, and cartridge heads load differently.

Start with the head type, not the line

Putting trimmer line on a string trimmer is not one job. On some tools you rewind a removable spool. On others you push a single length of monofilament through the housing and twist. Some heads take short pre-cut pieces that lock into slots. A few use a sealed, pre-wound cartridge you replace as a unit. Follow a bump-feed winding method on the wrong head and the line will not stay seated, will not advance, or will jam the drive.

Look at the cutting head before you cut any line. The markings on the head, spool, and remaining stub are the authority, not a generic rewind you saw on a different tool.

What the common head families look like

• Bump-feed head. A rounded bump knob or button sits on the bottom. You tap that knob on the ground while the head is spinning so more line can slip out of the eyelets. Inside you typically find a spool, a retaining spring, and arrows or wind-this-way marks. One or two cutting tails exit through metal or plastic eyelets in the side of the housing.
• Auto-feed head. The outside may look similar, but the head advances line without a deliberate hard bump, or with a lighter tap. Some still use a user-wound spool. Others hide a mechanism that is not meant to be rewound with bulk line. If there is no obvious bump knob, do not assume you can treat the head like a standard bump spool.
• Fixed-line or pre-cut head. There is no winding spool, or only a simple holder. You insert short, equal lengths of line into line-lock slots or holes until they click or seat. The tails stick out like spokes. There is nothing to wind.
• Quick-load or through-the-eyelet head. You feed a single measured length of line in one eyelet and out the opposite eyelet, then twist a knob or the head itself to take up the slack. The spool often stays inside. Do not strip the head down unless that design clearly requires it.
• Pre-wound cartridge. A sealed or semi-sealed cassette swaps as a unit. If the head looks like a closed drum with no user spool and no service split, stop before you pry it apart.

Single-line heads send one tail out of one eyelet. Dual-line heads send two tails, usually opposite each other. From the outside, count the exits. Dual-line spools often have two channels or a divider so the sides cannot tangle. Loading that spool as if it were a single open drum is a common mistake.

Check the head, the spool face, the leftover line, and any label on the shaft or motor housing. Arrows, diameter stamps, and line-lock holes tell you what the designer expected. If those marks are worn off and you cannot tell a cartridge from a user-wound spool, check the tool’s manual or a matching replacement-head description before you force a generic rewind. Forcing the wrong method can crack the housing or lose a spring you cannot easily replace.

Safety stops before you open the head

Do not put your hands on the cutting head while the tool can still start. Isolate energy first, then wait until every rotating part is still.

• On a corded electric trimmer, unplug the cord from the outlet and from the tool if the cord detaches.
• On a battery model, remove the battery pack and keep it away from the tool so it cannot be clicked back in by accident.
• On a gas model, let the engine come to a complete stop and wait until the head is still. If you can remove the spark-plug boot without forcing parts, do that so a pull on the starter cannot fire the engine while your fingers are in the eyelets.

Do not work on a hot muffler or a head that is still coasting. Loading line is not a fuel-system repair. If you smell raw fuel or see a drip, stop and have the leak fixed before the tool goes back into service.

Even with power isolated, the head is a cut hazard. Trimmer line is stiff monofilament; a short stub can slice a fingertip. Metal eyelets and cracked plastic have sharp edges. Wear eye protection any time you pull old line, because a coiled remnant can spring free. Gloves help when the line is fused or the housing is grimy, but they do not replace disconnecting power.

Clear leftover debris from the guard and head. Packed grass can hide a cracked cap. If the debris is hot from recent use, wait. If the debris shield is cracked through, missing, or hanging loose, do not continue as if new line will make the tool safe. The guard is there because a spinning line throws stones, wire, and broken monofilament. Replace or properly repair the guard before any test spin.

Never point the head at people or pets during a later test. Treat the cutting path as live as soon as the tool can run.

Match line diameter, shape, and length to the head

The accepted line diameter is normally printed or stamped on the head, the spool, a label on the shaft, or in the manual. That marking should win over leftover line from another tool or whatever bulk monofilament is on the shelf. Retail packages often list several common diameters. Treat those names as examples you must verify, not as a size most trimmers take. There is no safe universal default.

Line that is much thicker than the eyelets and spool were designed for can refuse to pass the eyelets, bind against the housing, lock the bump mechanism, or make the drive work harder than it should. Line that is much thinner than the head expects may slip the anchor, break at the eyelet, or fail to trigger a bump or auto-feed system that needs a certain stiffness.

When the spool has fill marks or clear rim edges but no length number, use those marks. Cut enough monofilament to fill each channel up to the mark without burying the rims or stacking coils above the flanges. If you have no marks, wind until the coils sit evenly below the rim and the spool can still drop into the housing without rubbing. Do not chase a universal foot or meter length. Heads vary, and an overfilled spool is a leading cause of no-feed.

Round line is enough for most consumer loading jobs because it seats predictably in round eyelets and simple slots. Shaped line—square, star, twisted, and similar profiles—can sit differently in a square notch or a tight eyelet. That is a seating issue, not proof that one shape cuts better or lasts longer. If a shaped line will not slide through the eyelet or will not lie flat in the channel, use the profile the head was clearly designed to hold, or go back to a matching round diameter.

Do not reuse old line that is brittle, chalky, fused, or heat-welded into a lump. Leftover monofilament that has sat in the sun or been cooked inside a stalled head will snap or weld again. Start with fresh line of the marked diameter.

Remove the cap, spool, and old line

Open the head only far enough to see the winding path, the spring, and the eyelets. Many bump-feed caps release when you press tabs on opposite sides and lift. Others unscrew with the bump knob, or a center fastener holds the stack together. Turn fasteners the way the head’s marks indicate. There is no universal always-left rule.

If the housing looks like a sealed cartridge with no tabs, no user spool, and no service split line, stop. Prying a sealed unit usually breaks clips you cannot replace.

As parts come off, keep them in order. A retaining spring often sits under the spool or inside the bump knob. A washer or inner cover may sit above the spool. Photograph the stack before anything rolls into the grass. A missing spring is a common reason a freshly loaded head will not feed.

Pull leftover line out of the eyelets in the direction it already runs. Do not lever against thin plastic with a screwdriver if a gentle pull will do. If the line is welded into the eyelet, cut it flush and push the stub through from the inside after the spool is out. Forcing a fused wad can crack the housing.

Inspect while it is open. A cracked spool, a flattened or missing bump button, a hub that no longer turns freely, or an eyelet worn into a knife-edge will not be fixed by new line. A razor-sharp eyelet will keep cutting fresh monofilament. A broken spool flange will let coils jump channels. Those parts need replacement, not another rewind.

Before you discard the remnant, note the winding direction. Coils still on the hub show whether the old line ran clockwise or counterclockwise when viewed from the top. Match that direction if the arrows are worn off and you have no manual.

Wind or insert line on a common bump-feed spool

This is the core reload for a typical user-wound bump-feed head. Skip it if your head is a through-hole quick-load, a fixed-line holder, or a sealed cartridge.

Anchor the line

Find the anchor hole, slot, or split hook in the spool hub. For a single-line spool, push one end of the line into that anchor until it holds. For a dual-line spool, read the hub before you cut.

• Some dual-line designs take one long piece folded in half, with the fold hooked on a center split so each half fills its own channel.
• Others take two equal pieces, each anchored on its own side.

Do not mix those methods. If the spool has a divider, keep each side in its own channel from the first turn.

Wind in the marked direction

Wind tightly enough that the coils stay put when you set the spool down, but not so tight that you stretch or flatten the monofilament. Each layer should lie in the channel beside the last, like hose on a reel. Crossing coils is the mistake that looks fine until the head is closed. The crossing becomes a knot, the bump knob clicks, and nothing comes out.

Follow the arrows on the spool. Correct is the marked direction, not the direction that feels natural in your hand. If the arrows are gone, use the direction you noted from the old remnant, or the direction printed inside the cap. Winding opposite the feed mechanism is one of the most common reasons bump-feed line will not advance.

Park the tails and stop at the rim

Leave tails long enough to reach through the eyelets, past the housing, and still give you something to hold. You need enough free line to thread the head and to even the two sides later. You do not need a wild excess that will slap the guard on the first spin.

Many spools have temporary notch parks or line-lock slots on the rim. After winding, press each tail into a notch so the coil cannot spring open while you carry the spool back to the housing. Those notches are parking spots, not permanent locks. The line must be able to pull out of them when the cap goes on and the tails go through the eyelets.

Stop winding when the coils reach the fill marks or sit evenly below the rim. If the last layer piles above the flange, unwind a few turns. An overfilled spool drags on the cap, welds from friction, and refuses to bump.

Thread the eyelets so the line can still slide

Align each parked tail with an eyelet, drop the spool into the housing in the orientation the hub requires, then pull the tails out of the rim notches and through the eyelets. The line should move a little when you tug. If a tail is pinched under the spool or twisted around the hub, open it and reseat. On a dual-line head, pull both tails to similar lengths before you think about the cap. Unbalanced tails often feed poorly even when the wind itself is tidy.

Load quick-load, fixed-line, auto-feed, and cartridge heads

These are alternate families, not ranked versions of the same rewind. If the parts in your hand do not include a marked spool and a clear winding path, you are probably in the wrong method.

Quick-load, through-the-hole heads

You usually do not remove a spool. Cut a single length that can pass in one eyelet and out the other with enough extra to grip. Feed the line through until the two sides are roughly even. Then turn the knob or the head in the marked direction so the interior take-up winds the slack inside. Stop when the exposed tails are a usable cutting length and the knob turns without grinding. If the line will not enter, the diameter is wrong or a broken stub is still in the passage.

Fixed-line heads

Use pre-cut pieces of the same length and the marked diameter. Push each piece into its line-lock slot or hole until it is fully seated. A piece that is only halfway in will fly out on the first spin. Both pieces—or all of them, if the head takes more than two—should be the same length so the head stays balanced. Do not wind leftover bulk line around a fixed-line holder that has no spool channel.

Auto-feed cassettes and pre-wound cartridges

If there is no user-accessible spool, the correct action is a matching refill cassette or a complete replacement head, not a homemade rewind. Do not cut the drum open to wrap random line around whatever is inside. You can destroy the feed spring or the one-way mechanism, and the head may never meter line again. Replacing the whole cassette is safer than opening it when the unit was sold as a sealed part. Not every auto-feed head is user-serviceable.

The typical mistake is treating every head like a bump-feed spool: prying off a non-serviceable cap, winding line in open space with no channel, or stuffing two diameters onto a hub that was never meant to be user-wound.

Reassemble the head and test the feed

Put the stack back in the order you photographed. The retaining spring has to sit where it can press the spool or the bump button. The spool has to engage the hub or drive dogs so it can turn in the feed direction. The cap has to catch every tab or full thread. A cap that is only half locked will come off in the grass. A spring that is cocked to one side will make the bump feel mushy or dead.

You know the cap is fully locked when the tabs have clicked, the threads have bottomed without cross-threading, and the bump knob—if present—can move the small amount it is supposed to move. If the cap will not sit flush, the spool is overfilled, a tail is trapped under a flange, or a part is upside down. Open it. Do not hammer the cap on.

Even the exposed cutting tails. Use the cut-off blade on the guard if the tool has one and the manual describes using it after a test spin, or trim the two sides so they match each other and stay inside the guard’s envelope. There is no universal stick-out number that fits every head. Follow any marks on the guard or the length that lets both tails clear the housing without looking wildly uneven. Dual-line heads need balanced tails.

With power still disconnected, do a hand check. Gently pull each tail to confirm the line can slide. On a bump-feed head, press the bump knob and see whether the spool can rotate a little and release a short amount of line. If nothing moves, the wind direction, a crossed coil, or a missing spring is still wrong. Fix that before any powered test.

Restore only the disconnect you removed: plug, battery, or spark-plug boot. Put on eye protection. Clear people, pets, toys, gravel, and loose wire from the cutting path. Hold the tool in a normal working stance with the head down and away from your legs. Start the tool only after the head is fully closed. A brief test is enough to see whether line stays in and whether a bump or auto-feed releases more line. Do not disable the guard to see better.

Stop immediately if you hear grinding or a high squeal, see smoke or smell melted plastic, feel the head wobble, or notice that the line has welded in place and the motor is straining. Shut the tool down, isolate power again, and open the head. Those symptoms are reasons to inspect, not reasons to keep running.

If the line will not stay in or will not advance

Most feed failures after a reload are loading and assembly errors. A rewind does not always fix feeding, especially when a part is worn or missing.

• Line spits out as soon as the head is closed. The tails were not seated in the eyelets, the anchor let go, a fixed-line piece was not fully locked, or the cap is not holding the spool. Open the head and confirm each tail actually passes through an eyelet and that pre-cut pieces are clicked home.
• The bump knob clicks but no line emerges. Look for crossed coils, the wrong winding direction, an overfilled spool dragging on the cap, or line welded from an earlier stall. The click is the button moving. It does not guarantee the spool can turn.
• Only one side of a dual-line head feeds. One channel may be crossed, one tail may be trapped under the spool, or the two sides may be badly uneven. Rewind both channels instead of pulling harder on the dead side.
• Line inside the spool is fused, melted, or welded. The line often sat still while the head kept turning, the spool was too full, the diameter was too large for the eyelets, or the head was held in dirt or against a solid edge until friction heated the monofilament. That is a mechanical heat problem, not a reliable electrical diagnosis. Cut out the welded mass and start over with fresh line of the correct diameter after you confirm the spool can spin freely.

Crossed coils often present as a head that worked for a few seconds and then locked. Wrong wind direction presents as a bump that does nothing even with a loosely filled spool. Oversized line presents as a tail that will not pass an eyelet or a spool that will not drop into the housing. A missing spring presents as a floppy bump button and no reliable feed. A worn eyelet presents as line that frays and snaps at the exit even when the wind looks perfect.

When the spool is cracked, the spring is gone, the eyelet is cut through, or a sealed cartridge no longer meters, the next step is a new spool or head—not another rewind with the same damaged parts.

Avoid a few fixes that make things worse. Do not pry into a head that still has power connected. Do not oil the line path; lubricant collects grit and can glaze the monofilament. Do not force a thicker line so it lasts longer. Do not run the tool with the cap off to watch the spool turn.

Limits: when new line is the wrong fix

Stop using the trimmer if the shaft is bent, the head housing is cracked through, a mounting nut or clip will not hold, an eyelet is torn out, or the guard is missing or shattered. New monofilament will not restore a cutting path you cannot control.

Correct line can still fail if you routinely grind the tails against chain-link, concrete, dirt, or hidden wire. Those surfaces cut and melt line and can stall the head until the remaining coil welds. Change the cutting habit or the accessory. Do not keep packing in thicker line to win against a fence.

Do not rewind a sealed cartridge with random leftover diameters. Do not mix two diameters on one spool. Do not run the tool as an edger if the head and guard are not meant for that orientation. Do not start the trimmer with the head open.

If the housing, shaft, or fuel system is damaged, or if you cannot identify the head type from the marks on the tool, a qualified small-engine or outdoor-power technician is the safer next step.

Frequently asked questions

Do I have to take the spool out every time?

Not always. Quick-load through-the-eyelet heads are meant to take a single length without a full teardown. Cartridge heads are meant to be swapped. A conventional bump-feed spool usually does come out so you can wind in the marked channels and keep coils from crossing.

Why did the line work for a minute and then stop?

A short successful run followed by a dead bump often means coils crossed after the first layer slipped, the spool was slightly overfilled and heated, or one dual-line tail pulled shorter than the other. Isolate power and open the head before a weld sets.

Can I use leftover line from a different trimmer?

Only if the diameter matches the marks on this head and the line is still flexible. A leftover coil from another tool is not close enough when it is thicker than the eyelets or already brittle.

The bump button feels loose after I put the cap back on. What did I miss?

A loose or mushy bump knob often means the retaining spring is missing, upside down, or not seated on the hub. Photograph the stack on the next teardown and compare. New line will not restore feed if that spring is gone.