How Do You Thread a Strimmer? Step-by-Step Guide for Every Spool Type

Quick Answer: How to Thread a Strimmer

Switch off the strimmer, remove the battery or kill the engine, then press the side tabs to release the spool head. Cut fresh monofilament line to the length stamped inside the housing, matching the diameter in your manual (typically 1.2 mm–2.4 mm), wind it in the direction shown by the arrow on the spool, snap the spool back into the head, feed each line end through its eyelet, and tug firmly until it seats. The exact steps depend on your spool type — use the section below to find your path.

Which Spool Type Do You Have? Choose Your Path

Threading steps differ significantly by spool design. Identify yours before reading further.

Corded electric, cordless/battery, and petrol strimmers can use any of the spool types above. The power source affects the safety steps before you start, not the threading method itself.

Safety Checklist Before You Start

Complete every item on this list regardless of spool type or power source.

• Corded electric strimmer: Unplug the mains cable from the socket. Do not rely on the power switch alone.
• Cordless/battery strimmer: Remove the battery pack from the tool before touching the spool head.
• Petrol strimmer: Move the kill switch to the OFF position, disconnect the spark plug lead, and allow the engine to cool fully before handling.
• Gloves: Wear cut-resistant gloves. Fresh trimmer line has sharp edges and the spool housing may have burrs.
• Guard: Leave the blade guard in place throughout. The guard does not interfere with spool removal and protects against accidental line release.
• Work surface: Work on a flat, stable surface in good light. Small spool springs and retaining caps are easy to lose on uneven ground.

How to Choose the Right Trimmer Line Gauge and Type

Using the wrong line diameter is the most common cause of feed failure after rethreading. Too thick and the line will not advance through the eyelets or will strain the motor; too thin and it snaps immediately on contact with dense vegetation.

Line Diameter by Strimmer Class

Always verify the diameter stamped inside your spool housing or listed in your owner’s manual. The table above shows typical ranges; the definitive reference is your specific model’s specification.

Line Profile: Round, Square, or Star-Shaped?

• Round: The standard profile. Quiet, long-lasting, suits most domestic lawns and light edges.
• Square / twisted: Cuts more aggressively through thick grass and weeds. Produces more noise and wears faster.
• Star-shaped / multi-sided: Maximises cutting edges per rotation. Best for heavy vegetation but puts more load on the motor — not recommended for lightweight cordless models.

Third-party monofilament line works in most heads provided the diameter matches. Some manufacturers state in their manuals that only branded line is approved; using third-party line in those cases may affect any warranty claim.

How to Thread a Bump-Feed Strimmer Spool (Single-Line, Step-by-Step)

1. Complete the safety checklist above. Confirm power is fully isolated.
2. Remove the spool head. Most bump-feed heads release by pressing two opposing side tabs simultaneously and pulling the cap downward. Some models use a retaining cap that unscrews counter-clockwise (left-hand thread is common — check your manual). Press firmly and evenly; forcing one tab snaps the housing.
3. Remove the old spool. Lift the spool out of the head. Note whether a small spring sits beneath it — keep this safe. The spring is required for the bump-feed mechanism to work.
4. Clear any jammed line. Remove all remaining old line from the spool. Check the eyelet channel in the head for fragments.
5. Cut fresh line to length. The maximum line length is stamped or moulded inside the spool housing. Cut to that length. Overloading the spool causes jamming; too short reduces cutting time between refills. A typical single-line spool takes 3 m–9 m depending on head size.
6. Find the anchor hole. The spool has a small hole or slot near its centre hub — the anchor hole. Fold one end of the line into a small hook or knot (or simply insert it 10 mm into the slot), then lock it into the anchor hole.
7. Wind in the correct direction. Look for the arrow moulded onto the spool face. Wind the line in that direction, keeping turns flat, tight, and even. Do not cross or overlap layers — crossed line jams during use. Leave approximately 150 mm–200 mm of line free at the end.
8. Secure the free end temporarily. Most spools have a small notch on the outer rim — clip the free line end into this notch to stop it unwinding while you refit the spool.
9. Reseat the spool. Place the spring (if present) back into the head first, then press the spool down onto the spring. The spool should sit flat and rotate slightly when pressed.
10. Feed the line through the eyelet. Pull the free line end out of the notch and guide it through the eyelet (the small hole in the outer housing). Pull until you feel or hear the line seat against the bump-feed notch inside the head.
11. Refit the retaining cap. Align the cap tabs with the housing slots and press firmly until both sides click. If the cap unscrews, turn clockwise (or the direction specified in your manual).
12. Test the bump-feed. Hold the strimmer above a hard floor and tap the head firmly downward. The line should advance 20 mm–30 mm with each tap. If nothing advances, the spool is seated upside down or the spring is missing.

How to Thread a Dual-Line Bump-Feed Spool

Dual-line spools follow the same removal and refitting steps as single-line. The winding stage differs.

1. Cut two equal lengths of line to the length specified in the spool housing. Equal lengths ensure both lines wear at the same rate and keep the head balanced.
2. Most dual-line spools divide the hub into two chambers with a central divider. Each line has its own anchor hole — one per chamber. Insert and lock each line end into its respective anchor hole.
3. Wind each line in the direction shown by the arrow on its chamber. On most designs both lines wind in the same direction; on some models they wind in opposite directions. Follow the arrow on the spool — do not assume.
4. Keep the two lines strictly within their own chambers. If a line crosses the divider, it will jam or snap at the divider edge during use.
5. Clip both free ends into the outer rim notches, then feed each end through the corresponding eyelet on opposite sides of the housing when refitting the cap.
6. After fitting, pull both lines to equal exposed length — typically 100 mm–150 mm beyond the housing, trimmed level by the blade guard’s cutting blade on first use.

How to Thread an Auto-Feed Strimmer

Auto-feed heads sense line length and advance new line automatically during use. They do not use a manually wound spool in the same way — the line feeds from a central cassette or is inserted directly into the rotating head.

Cassette-Type Auto-Feed

1. Purchase a replacement pre-wound cassette that matches your model (brand and model number matter — cassettes are not universal).
2. Remove the old cassette by pressing the release tabs or unscrewing the retaining ring.
3. Drop the new cassette into the head, align the line ends with the eyelets, and press until the cassette locks.
4. Pull each line end gently through its eyelet until it clears the housing by 100 mm–150 mm.

Direct-Feed Auto-Feed Head

1. Cut a single length of line to the diameter specified (verify in the manual — auto-feed heads are more sensitive to incorrect gauge than bump-feed heads).
2. With the strimmer off and battery removed, locate the two feed holes on opposite sides of the rotating head.
3. Insert one end of the line into one feed hole and push steadily until it exits from the opposite hole. The head’s internal channel guides the line through automatically — do not force it.
4. Centre the line so equal lengths extend from each hole, then restore power.
5. Run the strimmer briefly at low speed; the auto-feed mechanism will tension the line and trim it to operating length against the guard blade.

If the auto-feed strimmer consumes line rapidly after refilling, the most common cause is using line that is too thin for the head diameter, or a worn line-tensioning spring inside the cassette housing. In the latter case, replace the cassette rather than the line alone.

How to Replace Line on a Fixed-Line Head Strimmer

A fixed-line head holds individual pre-cut line segments in slots around a rotating disc. There is no spool to wind. When segments wear down, each is replaced individually.

1. Remove the battery or isolate power.
2. Inspect the rotating head. Each line segment is held in a slot or eyelet by a friction clip or a folded anchor. Identify how many slots are present (typically 6–12).
3. Pull the worn segment out of its slot. If it is fused or stuck, use pliers to grip and extract — do not pry against the head body.
4. Cut a replacement segment to the length specified by the head manufacturer. Segment length is not universal — consult the head’s instruction sheet or manufacturer’s website. Using segments that are too long causes the head to vibrate and puts lateral load on the motor shaft bearing.
5. Fold the segment in half and press the folded midpoint into the slot until the friction clip engages, so equal lengths extend from each side. Alternatively, on anchor-hole designs, insert the segment as a single strand and push it through until it centres.
6. Repeat for all worn slots. Replace all segments at once to maintain head balance.

Can you convert a fixed-line head to a bump-feed head? Many strimmer shafts use a standard UNC thread (typically 1 mm left-hand thread), so aftermarket bump-feed heads do physically fit. Check thread size and rotation direction before purchasing a replacement head. Not all motors are rated for the increased drag of a bump-feed system.

Common Threading Problems and How to Fix Them

Line breaks immediately after rethreading

The line diameter is too thin for the vegetation being cut, or the line was wound with crossed layers that create stress points. Re-examine the spool for layer crossings and move to the next diameter up if the vegetation is dense.

Line will not advance when bumping the head

Check three things in order: (1) the spool spring is present and seated correctly; (2) the spool is the right way up — most are marked TOP or have an arrow; (3) the free line end is actually fed through the eyelet and not still clipped in the rim notch.

Trimmer line has melted or fused (welded) inside the spool

Line welding is caused by frictional heat generated when the line stalls against the housing during use — typically because the line was wound in too many layers, was wound in the wrong direction, or the incorrect diameter was used. The only fix is to discard the fused spool, clear all debris from the head, and rewind with the correct gauge wound in a single layer in the correct direction.

Spool cap keeps popping off during use

The retaining tabs are worn or broken, or the cap was not fully pressed home when refitting. Inspect the tabs for cracks; if damaged, replace the cap assembly. A temporary fix is to ensure the line is not packed so tightly that the spool pushes against the cap from inside.

Line tangles inside the head instead of feeding out

The line was wound unevenly or in crossed layers. Unwind completely, clean the spool, and rewind flat and tight in the direction of the arrow. Ensure the line does not exceed the maximum length stamped in the housing — overfilling is the primary cause of internal tangles.

Can I use pre-wound replacement spools instead of threading my own?

Yes. Pre-wound replacement spools are available for most popular bump-feed heads (Bosch, Flymo, Black+Decker, and others sell model-specific units). They remove the risk of incorrect winding direction or overcrowded layers. The trade-off is cost per metre of line compared with buying bulk monofilament and winding your own.

Frequently Asked Questions

What diameter trimmer line should I use for a cordless strimmer?

Most lightweight cordless strimmers (18 V–20 V) specify 1.2 mm–1.6 mm monofilament. Mid-range 36 V–40 V models typically accept 1.6 mm–2.0 mm. The definitive reference is the diameter stamped inside your spool housing or stated in the owner’s manual — not the line packaging.

Which direction do I wind the line onto a strimmer spool?

Wind in the direction indicated by the arrow moulded on the spool face. Winding direction varies by model — there is no universal rule. Winding in the wrong direction causes the line to lock up inside the head rather than feeding out.

Why does my strimmer line keep breaking so quickly?

The three most common causes are: using a line diameter that is too thin for the cutting conditions; crossed or uneven winding that creates stress points; or running the cutting line too short so the line tip strikes the guard housing repeatedly. Check diameter, rewind evenly, and ensure adequate line length extends beyond the housing before use.

Do I have to use the manufacturer’s own brand of trimmer line?

Most heads accept any monofilament line of the correct diameter and profile. Some manufacturers state in their manuals that only branded line is approved, which may affect warranty claims. Third-party line that matches the specified diameter and is a round profile works reliably in the majority of domestic bump-feed and auto-feed heads.