Threading a weedwhacker means loading fresh trimmer line into the cutting head so the tool can feed and cut again. Stop and isolate the tool, identify whether you have a bump-feed spool, a feed-through or quick-load head, or a fixed-line design, then wind or insert the line size that head specifies. Reassemble it, leave only a short working length outside the eyelets, and test the feed in a clear area.

What threading a weedwhacker means

A weedwhacker and a string trimmer are the same class of consumer tool: a handheld cutter that spins monofilament line, or a similar nylon cutting line, to trim grass and light weeds. Threading, restringing, or reloading the machine is not an engine or motor repair. It is the job of putting fresh trimmer line into the trimmer head so the head can pay out line and cut again.

That job can look like three different tasks, depending on the head:

  • Removing an inner spool, winding line onto it, and putting the spool back.
  • Pushing a length of line through a preloaded or quick-load head and taking up slack with a knob, without fully unwinding a spool by hand.
  • Seating short, pre-cut pieces in a fixed-line head that does not wind at all.

This article covers those common consumer replaceable-line heads in general terms. It does not cover metal brushcutter blades, commercial saw heads, sealed proprietary cartridges that you replace as a unit, or any method that requires drilling, grinding, or modifying the head or cutting guard. Head designs differ. If the parts in your hand do not match a simple spool, through-hole, or fixed-slot layout, stop and use the instructions for that specific head.

Safety steps before you open the head

Treat the trimmer as a machine that can start, spin, or spring apart until you have isolated it. Do the isolation steps that match the power source, then wait until moving parts have stopped completely.

  • Battery models: Switch the tool off, remove the battery pack, and keep the pack away from the work area so it cannot be clicked back in by accident.
  • Corded electric models: Switch the tool off and unplug it from the outlet. Do not rely on a trigger lock alone.
  • Gas models: Switch the tool off and set it down so it cannot roll or tip. If you are comfortable doing so and the manufacturer instructions allow it, disconnect the spark-plug lead so a pull on the starter cannot fire the engine. Do not open the fuel system or service the carburetor for a line change.

Residual hazards remain after shutdown. Gas engines and mufflers can stay hot enough to burn skin. Fuel on the housing is a fire and slip hazard, so wipe obvious spills and work away from ignition sources. Bump-feed heads often contain a compressed spring; the cap, knob, or spool can jump free when the last latch releases. Old line stubs, grit, and cracked plastic can cut fingers or flick debris toward your face.

Wear safety glasses. Close-fitting gloves help with sharp stubs and hot housings without getting caught in parts. Tie back long hair, skip loose sleeves and dangling jewelry, and keep bystanders and pets out of the work area. If the head is cracked, the spring will not stay in its pocket, or you cannot isolate the tool confidently, stop and have a qualified repair shop or the manufacturer support channel handle it.

Identify the trimmer head and spool type

The correct threading method depends on the head you actually have, not on a generic picture of a spool. Look at the bottom and sides of the trimmer head before you force anything open.

Bump-feed heads

A bump-feed head usually has a rounded knob or button on the bottom. During use, tapping that knob on the ground is meant to release a short length of line. Inside, you will typically find a removable inner spool, a spring under or around the spool, and a cap or cover that twists, clips, or unscrews. Many of these heads are dual-line: two ends of line exit through two eyelets on opposite sides of the housing. Some are single-line, with one exit.

Auto-feed heads

An auto-feed head may look similar from the outside but is designed to pay out line during operation without a deliberate bump. The inner layout can still be a wound spool, a cartridge, or a feed-through path. If there is no obvious bump knob and the cover has no winding arrows, do not assume it winds the same way as a bump head.

Feed-through and quick-load heads

These heads are built so you pass a single length of line through the housing, often through a visible tunnel or aligned eyelets, then twist a knob or click a retainer to take up slack. You may not need to lift out a full spool. Look for a through-hole, arrows printed across the cap, or a knob that turns independently of the whole housing.

Fixed-line heads

A fixed-line head does not store a long coil. It holds short, pre-cut pieces in slots, clamps, or eyelets. There is no winding direction because there is no winding. If you see several discrete pockets around the rim and no inner reel, you are not looking at a bump-feed spool job.

Other useful cues: arrows or the words for winding direction molded into the spool, a divider wall that splits the spool into two chambers, holding notches on the spool rim, and whether the inner reel lifts out as one piece. Single-line layouts have one exit path. Dual-line layouts have two eyelets and usually two wrapped chambers or one length folded in the middle. If nothing on the head shows a path, treat that as a signal to use the manufacturer diagram rather than guess.

Choose line that matches the head

Use the line diameter and single- versus dual-line setup specified for that head. Look first at markings on the head, the inner spool, the shaft label, or the tool’s manufacturer instructions. Aftermarket line packaging often lists several diameters; that does not mean every diameter fits your eyelets or spool.

Line that is too thick can jam in the eyelets, refuse to feed, overheat the spool, or strain the drive. Line that is too thin breaks sooner, nests loosely, and can weld to itself when the head gets hot from friction. Stiffness and shape matter as well. Round monofilament line is the common default for a basic reload. Twisted, serrated, or other shaped line exists, but it is not required to thread the head, and a shape that is too aggressive for a small eyelet can bind the same way oversized round line does.

Do not use a universal cut length. If the head or instructions state a length, follow that. If they do not, cut only as much as will fill the spool in even layers without climbing over the rim, blocking the cap, or burying the holding notches. Leave short working ends to park in those notches and pass through the eyelets. Extra line stuffed into a full spool is a common cause of no-feed and melted coils, not a way to mow longer between reloads.

Match the layout. A dual-line head needs two usable ends exiting the housing. A single-line head needs one. Do not double a single-line head or leave one eyelet empty on a dual-line head unless the instructions for that head say to. If the old line is still readable, use it only as a rough clue for diameter and then confirm against the markings on the current head. Worn, oval eyelets can make even the correct diameter behave like the wrong one.

How to load a common bump-feed or removable spool

This is the method most homeowners need when the inner reel lifts out. It is not the method for a sealed cartridge or a fixed-line head. Work on a bench or a clean patch of ground so the spring and small parts cannot disappear into grass.

  1. Note the parts order before anything flies apart. Look at how the cap locks, which way the bump knob faces, and where the spring sits. Many heads use a spring in a pocket under the spool. Photographing the stack with a phone is useful if you have never opened that head.
  2. Open the head. Twist, unclip, or unscrew the cover the way the latches are designed to move. Do not pry with a screwdriver if a molded tab is meant to release by hand. Keep a thumb over the cover as the last latch lets go so the spring cannot launch the knob.
  3. Lift out the spool and clear debris. Remove leftover line, packed grass, and grit from the housing, eyelets, and spool grooves. A blocked eyelet will cut new line as soon as the head spins. If the spring comes out, set it aside in the same orientation you found it. Do not stretch or invert it to “make it bump better.”
  4. Inspect the parts. Replace a cracked housing, a flattened or missing spring, or an eyelet worn into a sharp oval. Forcing new line through damaged parts repeats the failure.
  5. Cut a manageable length of the correct line. Use the diameter marked for the head. If no length is marked, start with enough to fill the spool in neat layers and stop before the coil stands above the rim. You can always wind a second, shorter refill later. Avoid one giant coil that has to be forced under the cap.
  6. Anchor the line on the spool. Most reusable spools have a hole, slot, or hook for the midpoint or for each end. For a dual-line spool with a divider, seat the midpoint in the center slot so each half of the line can occupy its own chamber, or seat two separate pieces if that is how the spool is molded. If your spool has a divider or two arrows, treat each side as a separate wrap.
  7. Wind tightly and evenly in the marked direction. Follow the arrows on the spool. Winding against the marked winding direction is one of the most common reasons a bump-feed head will not feed. Keep the line snug so wraps lie side by side instead of crossing. Crossing and loose wraps expand when the head spins, then lock against the housing.
  8. Park the last few inches in the holding notches. Leave short tails in the rim notches or clips so the coil cannot unwind while you carry the spool back to the housing. The tails should be long enough to reach through the eyelets, not long enough to tangle around the spring.
  9. Set the spool back in the housing. Align the parked tails with the eyelets, feed each tail through its eyelet, then seat the spool so it sits flat. If the head uses a spring, the spring must sit in its pocket before the spool or cap goes on. A cocked spring will pop the cap off or refuse to bump.

Some bump-style heads use a hybrid feed-through method: you push line through the housing first, then wind by turning the bump knob instead of wrapping the removed spool by hand. That is not the same as open-spool winding. If the inner reel does not want to come out, or the cap is clearly a winding knob with a through-channel, switch to the feed-through sequence below rather than prying the reel free.

Feed-through, quick-load, and fixed-line heads

These designs are common on newer consumer trimmers. They are not interchangeable with hand-winding a removable spool. Use the path the head already shows.

Feed-through or quick-load heads

Align a single length of the correct line with the through-hole or with the arrows across the cap. Push the line until roughly equal amounts exit each eyelet on a dual-exit head, or until the amount on a marked single-exit head matches the marks on the housing. Then twist or click the knob only as that head is designed to take up slack. Stop when the coil is snug and the cap is fully seated. Do not keep cranking after the knob gets hard to turn; that overfills the chamber and welds the line.

If the line will not pass, do not hammer it through. Recheck diameter, clear the tunnel, and confirm both eyelets are in line. Some heads only accept line when the knob is in a load position. If you cannot find that position from the molded marks, use the head diagram instead of forcing the knob past a stop.

Fixed-line heads

Use only pre-cut pieces in the length and diameter the slots or eyelets are built for. Seat each piece fully so both cutting ends, or the single working end if that is the design, are captured by the retainer. A piece that is only halfway in can fly out when the head reaches speed. Do not try to wind leftover spool line into a fixed-line head, and do not stack extra pieces in a slot that is sized for one.

Stop improvising when the head uses unusual locking tabs, a sealed cartridge, or a path you cannot see from the outside. Those are the cases where the manufacturer instructions for that head take priority over any general sequence.

Reassemble, set line length, and test the feed

A correctly seated head looks closed all the way around and does not rattle through a half-latch. The cap or cover must be fully locked. On a dual-exit head, line must pass freely through both eyelets. Tug each tail gently to confirm it is not pinched under the spool rim. Rotate the spool or knob by hand, with the tool still isolated, and confirm the line can move a little without grinding.

Trim the protruding line so it cannot hit the cutting guard before the head spins up. Many guards also act as a cutoff once the tool is running, but you should not start with tails so long that they slap the shield, the shaft, or your clothing. There is no single stick-out measurement that fits every head and guard. Leave a short working length, then let the guard trim any excess on the first spin if that is how your shield is shaped.

Reverse the isolation steps only when the head is closed and the work area is clear. Reinstall the battery, plug in a corded tool, or reconnect a spark-plug lead you removed. Keep bystanders, debris, and loose clothing away. Stand so you are not in the cutting plane of the line. Start the trimmer and, for a bump-feed head, tap the knob on bare soil or a clear patch of turf, not on rock, concrete, or metal. One firm tap should release a short increment if the wind and assembly are correct. Auto-feed heads may pay out during a brief run without a tap.

The first bump does not always feed. If nothing comes out, shut the tool down and isolate it again before you reopen the head. Do not raise the speed, lean into pavement, or keep slamming the knob to force a jam clear. Those habits break knobs, flatten springs, and melt nylon.

When you should use the tool or head manual instead

A general consumer method is a starting point, not a substitute for the diagram molded on an unusual head. Use the manufacturer instructions when any of the following is true:

  • The head has no arrows, slots, or visible line path.
  • The cover is a sealed cartridge or a one-piece replacement, not a serviceable spool.
  • Locking tabs, bayonet lugs, or a multi-piece bumper do not match a simple twist-off cap.
  • The trimmer is a commercial unit, a multi-tool power head with an uncommon attachment, or a head that also accepts a blade.
  • You cannot tell whether the spring belongs above or below the spool, or the spool will not sit flat without force.

Wrong line diameter and winding against the marked direction are everyday causes of no-feed, melted spools, and clutch or motor strain. Forcing the wrong method can crack the housing or throw the cap. Do not enlarge eyelets, drill the spool, grind the guard, or bypass retainers to make thick line fit. Those changes can let line or broken plastic leave the head at speed.

A replacement head or a prewound spool is the better option when the housing is cracked, the eyelets are egg-shaped, the spring is weak, or you cannot identify the design. Do not assume an aftermarket head is a drop-in fit for every shaft and drive. Match the mounting, the line capacity, and the instructions that come with the replacement. If the tool is still under a warranty that requires original parts or procedures, follow that paperwork rather than improvising.

Troubleshooting line that will not feed or keeps breaking

After a careful reload, most feed problems are setup errors or worn parts, not a mysterious defect in every trimmer. Isolate the tool again before you open the head.

The head will not pay out line

  • The coil was wound against the arrows.
  • Wraps are loose, crossed, or stacked above the spool rim.
  • Tails were never seated in the holding notches, so the coil slumped in the housing.
  • The line diameter is too large for the eyelets or too small to stay in the grooves.
  • Grass, sand, or old fused nylon is blocking an eyelet or the bump knob.
  • The spring is missing, inverted, or not in its pocket, so the bump mechanism cannot release.
  • The cap is only half-latched and is dragging on the spool.

Line welds, snaps at the eyelet, or unwinds inside the housing

Heat from friction can fuse nylon in an overfilled or poorly wound spool. That is a common field issue when wraps cross, the cap rubs the coil, or the operator keeps bumping a jammed head. Snapping at the eyelet often means the eyelet is worn sharp, the line is too thin or brittle from age and sun, or the tail is exiting at a kink. Line that unwinds into a bird’s nest inside the housing usually was not parked in the notches or was wound so loosely that centrifugal force collapsed the coil.

Do not soak a welded spool in solvents or household chemicals to “save” the nylon. Cut the fused mass out, clean the parts, and rewind with the correct diameter in the marked direction. Replace line that has sat in sun or heat until it is chalky and brittle. Replace a cracked housing, a weak spring, or an egg-shaped eyelet instead of pushing more line through the same damage.

If the trimmer runs but the head still will not feed after a clean rewind, the problem may be a worn bumper, a damaged inner reel, or a drive issue beyond threading. That is the point to stop repeating the same reload and consult the tool instructions or a qualified repair shop.

A practical order of work

Keep the job in this order so you do not skip the step that actually matches your head:

  1. Stop the weedwhacker and isolate the battery, cord, or ignition.
  2. Put on eye protection and clear the bench.
  3. Identify bump-feed, auto-feed, feed-through or quick-load, or fixed-line construction from the knob, arrows, through-hole, or slots.
  4. Confirm line diameter and single- versus dual-line layout from the head or manufacturer markings.
  5. Open only as far as that design requires. Wind a removable spool, feed line through a quick-load path, or seat pre-cut pieces.
  6. Reassemble with the spring in its pocket if the head uses one, lock the cap, and trim the tails so they clear the cutting guard.
  7. Restore power, test feed in a clear area, and shut the tool down again if the line does not move.

Threading a string trimmer is a maintenance skill, not a universal trick. The safe sequence is always the same at the start and the end: isolate the tool, match the method to the trimmer head you have, use the specified monofilament line, and test away from people. When the head in your hands does not match a common spool or feed-through layout, the markings on that head are the instructions that matter.