To refill a typical grass or string trimmer, make the tool safe, open the rewindable (often bump-feed) head, remove leftover line, load new line in the diameter the head is built for, wind it in the direction marked on the spool, thread the ends through the eyelets, reassemble, and leave a short working length ready to cut. Some heads instead take pre-cut pieces or a drop-in replacement spool, so identify the head type before you start winding.

What “refill” means on different trimmer heads

Most consumer grass trimmers and string trimmers cut with nylon trimmer line held in a plastic trimmer head, not with a metal blade. “Refill” is not one procedure. It usually means one of three jobs:

  • Rewind a reusable spool. You open a bump-feed or similar head, take out the spool, wind fresh line onto it, route the ends through the eyelets, and close the housing.
  • Insert pre-cut pieces. You push short lengths of line through slots or eyelets. There is no long winding session.
  • Swap a replacement spool or cartridge. You drop in a pre-wound spool, or you replace a sealed unit the manufacturer never intended you to rewind.

Look at the head before you cut a long piece of line. A rewindable bump-feed head often has a bump knob on the bottom, two (sometimes one) eyelets in the side wall, a two-piece cover that unclips or unscrews, and winding arrows on the spool or housing. An auto-feed head may look similar from the outside but advances line by a mechanism inside rather than by tapping the knob on the ground. Quick-load designs let you feed a length of line through the head and twist or wind without fully disassembling a spool. Fixed-line heads hold short pieces in slots or clamps and are not meant to store a long coil. A sealed cartridge or factory replacement spool may have no useful winding marks and no way to open the drum without breaking it.

None of those patterns is universal. Caps, tabs, and arrows differ by model. If the housing looks sealed, or the owner’s manual or packaging says to install a complete spool or head, do not pry the unit apart.

Gas, battery, and corded trimmers share the same line-loading task. The power source changes how you prevent an unintended start, not whether you wind clockwise or use dual-line. After the tool is locked out, the head work is the same family of steps.

Make the trimmer safe before you open the head

A spinning head can throw line and debris. A tool that starts while your hands are on the housing can cut skin or send the shaft moving. Treat lockout as the first step, not an afterthought.

  1. Wait until the cutting head has fully stopped. Do not grab a still-coasting bump knob or eyelets.
  2. Keep the switch or trigger off and keep bystanders and pets away from the work area.
  3. On a battery model, remove the battery pack and set it aside. Do not rely on the trigger lock alone.
  4. On a corded model, unplug the power cord from the outlet or extension cord so the tool cannot start if the switch is bumped.
  5. On a gas model, shut the engine off and wait until the head is still. Let a hot engine and muffler cool before you rest the tool on your lap or work close to the powerhead. Follow the owner’s manual for any additional lockout the maker specifies. Do not invent extra fuel-system or ignition service for a line change.

Wear eye and face protection. Whipping line, grit, and bits of old nylon can fly as soon as you later test the feed. Gloves help when you pull fused line or handle a dirty head. Long sleeves and closed shoes are reasonable around a tool that still has a cutting guard and leftover line stubs.

If the head is packed with wet grass, wipe or pick out the debris after lockout so plastic tabs are visible and you are not forcing a cover against a wad of vegetation. If the engine or motor housing is hot, wait. A line refill is not an emergency repair.

Follow the labels on your tool. If a head, spring, or fastener is damaged in a way that makes the housing unsafe to close, stop and replace the head or have a qualified service shop do it rather than taping a cracked drum back together.

Gather the line and a few simple tools

You rarely need a workshop for a consumer head. Typical items are enough:

  • Safety glasses or a face shield, plus gloves
  • Scissors, sidecutters, or a dedicated line cutter
  • A clean cloth for grass and grit
  • Whatever simple fastener the cover uses—often none beyond molded tabs, sometimes a hand-turned knob or a common screwdriver if a retainer is present

Before you buy line, read what is already on the tool. Many heads, spools, or leftover packaging show a diameter range. The owner’s manual, if you still have it, is the first place to confirm that range. Matching diameter matters more than matching a marketing name. Line that is too thick can jam in the eyelets, keep the cap from seating, or refuse to feed. Line that is too thin can break at the eyelet, slip in the channels, or feed poorly.

Inspect the old spool while you still have it. Note whether it is single-line or dual-line, whether it has a divider wall or two separate channels, and whether there is a mid-point slot or anchor hole. Those features tell you how to fold or split the new piece. Also note winding arrows and any “this way” marks. Photograph the assembled head and the empty spool if the layout is unfamiliar; that snapshot is more useful than guessing later.

Line shape—round, twisted, serrated, or other profiles—changes cutting feel and noise for many users, but it does not replace a compatible diameter or a head designed for that line. Do not assume a different profile will feed in a head that is already marginal, and do not treat any one shape as universally better. If the head or manual limits you to a listed type, stay inside that limit.

Work on a bench or a clean patch of ground with a tray or box for small parts. Bump knobs, springs, and washers are easy to lose in grass. Keep the cutting guard on the tool unless the manual requires you to remove it for access; the guard and any line-cutoff blade are part of how the head is meant to run.

Open a typical head and remove the old line

Common rewindable covers come off in a few familiar ways. Yours may use one of them, or a close variation:

  • Squeeze opposing tabs on the cover and lift the cap off the housing.
  • Hold the housing and unscrew a cap or bump-knob assembly (direction is whatever the marks or threads on your head indicate—do not assume every brand unscrews the same way).
  • Press release latches and lift the bump knob and spool together, watching for a spring underneath.

Follow the marks molded into the head. If a cover will not yield, stop and look again rather than prying with a screwdriver against thin plastic. Forced tabs crack, and a cracked cover will not stay closed under vibration.

Lift the spool straight out. Note the order of parts: cover, spool, spring, and any washers or retainers. The spring in a bump-feed head is what lets the spool click and release line when you tap the bump knob. Set it where it cannot roll away.

Inspect before you rewind:

  • Housing and cover. Cracks, missing tabs, or a warped rim mean the head may not stay together.
  • Eyelets. These metal or plastic bushings in the side wall guide the working line. Sharp grooves, missing inserts, or egg-shaped holes chew line and cause breaks at the exit.
  • Spool. Check for broken divider walls, stripped hubs, and melted flanges.
  • Spring. A missing, bent, or collapsed spring is a common reason a bump-feed head will not advance line even after a perfect rewind.

Pull off leftover line. If the old line is merely short, unwind it. If it is welded into a lump from heat and friction, cut it in several places and peel the fused nylon off the hub rather than yanking until a flange snaps. Clear grit from the channels, the spring seat, and the inside of the cover. Line left in the sun or stored for a long time can become brittle; do not reuse a coil that already shows cracks or a chalky surface if you have fresh line available.

If the head is a type you cannot open without breaking it, skip ahead to replacement-spool or complete-head methods. Forcing a sealed drum is how people destroy a still-usable tool.

Choose line diameter and cut a workable length

Correct diameter is model-specific. Start with markings on the head, the old spool, the line packaging you already used successfully, or the owner’s manual. Hardware-store bins often list several diameters; “close enough” is a frequent cause of a cap that will not close or a head that will not feed.

Qualitatively:

  • Too thick can bind in the eyelets, overfill the drum, or keep the cover from seating.
  • Too thin can slip, break at worn eyelets, or fail to engage the feed mechanism.
  • Too long stacks into a bulky coil that rubs the housing, binds when you bump, or prevents the cap from locking.

Length is also model-specific. Dual-line jobs often use one piece folded at the midpoint so each half fills one channel, or two equal pieces if the spool has two anchors. Single-line spools take one continuous length in one channel. There is no single footage or metre count that fits every consumer head. If a package or manual gives a length for your head, use that. If you only have a leftover stub as a guide, cut a conservative piece you can still handle, wind until the drum is comfortably full without a proud mound, and stop. An example such as “about as much as the old coil held” is only an illustration—you must verify capacity on your spool. Overfilling is harder to fix after the cover is on than cutting a bit more line later.

Cut the ends clean and square so they start through eyelets easily. If you fold one piece for dual-line, find the midpoint before you wind so both sides stay even. Uneven halves make one side run out first and can unbalance the head.

Wind single-line and dual-line spools the right way

Direction is not a matter of preference. Most spools or housings have winding arrows or a “wind this way” mark. Those arrows exist because a bump-feed or auto-feed mechanism releases line in one direction. Winding against the mark is one of the most common reasons the line will not advance when you tap the bump knob.

General pattern for a single-line spool:

  1. Anchor the end in the hole, slot, or clip on the spool hub, or follow the anchor your spool actually has.
  2. Wind in the arrow direction, keeping the line snug and in even layers.
  3. Avoid large overlaps and crossed windings that can lock the coil so it cannot slip when the head tries to feed.
  4. Leave a working end long enough to reach through an eyelet plus a short cutting length. Many spools have a notch on the rim to park that end so the coil does not unwind in your hands.

General pattern for a dual-line spool:

  1. If the spool has a mid-point slot or hole, fold one piece of line in half, seat the fold in that slot, and wind each half in its own channel—still following the arrows, which are often the same rotational direction on both sides of a divider.
  2. If the spool takes two pieces, anchor each piece in its own hole or clip and fill each channel separately, keeping the lengths even.
  3. Use the divider wall. Do not let one side climb over into the other channel, or the two lines can weld or tangle.
  4. Park both working ends in the rim notches if the spool has them.

Keep tension firm enough that the coil does not slump, but do not stretch nylon until it thins or heat-glazes. Neat, side-by-side wraps feed more reliably than a rushed ball. You do not need a counted number of turns; you need a coil that stays below the flanges and still leaves room for the cover and spring to work.

Before the housing goes back on, route each working end through its eyelet or designated holder. Closing the head with the ends trapped inside is a common mistake. Align the parked ends with the eyelets, slide or drop the spool into the housing so those ends pass through, then pull a short amount of line out so it cannot slip back while you fit the cover.

Do not treat one brand’s printed diagram as if it applies to every head. If your spool has unusual posts, a through-hole for quick-load, or no divider, follow those features instead of forcing a dual-channel method onto a single-channel part.

Reassemble, set a starting length, and test the feed

Seat the spring (if the head uses one) in its pocket, then the spool, then the cover. The spool must sit fully down so the hub engages and the eyelets stay lined up with the exits you just threaded. If the cover has tabs, they should snap or screw into the same orientation they had when you opened the head. A bump knob should move slightly against the spring if that is how the design works; a knob that is jammed solid or that falls off is not correctly stacked.

If the housing will not close, do not hammer it. Typical causes are an overfilled spool, a coil that climbed above the flange, a spring out of its seat, a spool rotated so parked line is not at the eyelets, or windings in the wrong direction that bulk up against the cover. Open it, correct the stack, and try again. A cover that only stays on if you squeeze it will pop off in the grass.

Starting length should be short enough to stay controllable. Many cutting guards include a line-cutoff blade that trims excess the first time the head spins. Whether you have that blade, and how long a stub is safe, is model-dependent. Leave enough line to cut, not a long whip that can slap the shaft, the guard, or your legs. After lockout is removed and you are ready to test, the guard’s cutter—if present—can shorten proud ends. Do not stand in line with the plane of the spinning head while that happens.

Test only after the head is fully assembled, with eye and face protection on, in a clear area, and with the tool fully under control:

  • Reinstall the battery, plug in the cord, or start the gas engine using the normal, manual-specified starting routine—still keeping a firm grip and a balanced stance.
  • Bring the head up to cutting speed away from people, glass, and loose gravel.
  • For a bump-feed head, tap the bump knob on firm ground the way you would in the yard and see whether a short amount of line advances. Do not expect a fixed number of bumps or a promised feed distance; you only need to confirm that the coil can slip and that both dual-line ends (if any) appear.
  • For an auto-feed head, use the feed method the tool is designed for. Not every auto-feed unit works like a bump head, and some advance only while the head is spinning under load or at a change in speed.

If nothing comes out, shut the tool down, lock it out again, and go to troubleshooting rather than rapping the head harder. Repeated empty bumps can damage a knob or spring.

Other refill methods: pre-cut pieces, quick-load, and replacement spools

Rewinding is common, not mandatory. Use the method the head was built for.

Pre-cut or fixed-line pieces. Some heads never store a long coil. You cut short lengths (or buy pre-cut pieces sized for that head) and push each piece through an eyelet, slot, or clamp until it locks or seats equally on both sides. Refill means replacing those pieces when they wear back, not opening a spool. Forcing a long rewind into a fixed-line head wastes line and will not feed.

Quick-load through the head. Certain designs let you align arrows on the housing, push a single length of line straight through both eyelets, then twist the knob or rotate the spool to wind the line inward without removing the drum. The “wind” still follows marks on that head. If the through-hole will not accept the line, the diameter is probably wrong or the inner channel is not aligned.

Drop-in or sealed replacement spool. Many consumer heads are meant to take a pre-wound spool. You open the cover, lift out the empty spool, drop in the new one, and thread the factory-cut ends through the eyelets. Some cartridges are sealed; the intended refill is the whole cartridge or the entire trimmer head. If the manufacturer specifies a complete head, replace the head. Compatibility of an aftermarket spool with a named trimmer is not something to guess from appearance alone—use the designation on your housing or manual, or take the old spool to match, rather than assuming every “universal” drum will click into your spring and eyelets.

These alternatives are not lesser versions of a rewind. A sealed unit that still feeds is doing its job. Prying it open to save a small amount of nylon often costs the housing.

Troubleshooting poor feed and when to replace the head

When the line will not advance, the cause is often in the last rewind, not a mysterious failure of the motor or engine. Typical issues include:

  • Wound against the arrows. The mechanism tries to pay out line and instead tightens the coil. Rewind in the marked direction.
  • Overfill or crossed windings. The coil binds against the cover or against itself. Remove line until the flanges are clear and the wraps lie in even layers.
  • Wrong diameter. Thick line sticks in the eyelets; thin line may not catch. Return to the marked size.
  • Ends not through the eyelets. The head is closed on a trapped coil. Reopen and route the working ends out.
  • Debris or fused leftover line. Heat and friction can weld loosely wound or leftover nylon so it will not slip until you clear it.
  • Weak, missing, or unseated spring. A bump-feed head needs that spring to release the spool in small steps.
  • Worn eyelets. Sharp grooves act like a blade and snap line at the exit, which feels like “the line will not stay in.”
  • Brittle line. Nylon that sat in sun or sat for a long time in a dry shed can shatter as soon as it flexes. Fresh line of the correct diameter is the fix, not a harder bump.

Line that snaps repeatedly at the eyelet often points to a damaged bushing, a too-small diameter, or a cutoff blade and guard relationship that is nicking the line. Line that welds itself on the spool often points to leftover heat-glazed nylon, overlapping wraps, or running the head with almost no line out so the tips melt back into the drum.

Replace the head—or have it replaced—rather than forcing another rewind when you see a cracked housing, broken cover tabs, a destroyed eyelet that cannot be renewed, a hub that no longer drives the spool, or a bump knob that will not stay captured. Those parts keep the line inside a spinning drum. Tape, zip ties, and oversized line will not restore that function reliably.

If the powerhead itself will not run, that is a different problem from an empty spool. A line refill will not fix a dead battery, an unplugged cord, or a gas engine that needs service. Stay inside the line task unless you are qualified for the rest of the tool.

Once the head feeds cleanly, return to ordinary use: keep the guard in place, keep the cutoff blade (if fitted) free of packed grass, and refill again while you still have enough stub to see how the old coil was arranged. Identifying the head type, locking the tool out, matching diameter, winding with the arrows, and leaving the ends through the eyelets is the whole job for most consumer grass and string trimmers.