How Do You Replace String on a Stihl Weed Eater? (Complete Guide for All Head Types)

Which Stihl Trimmer Head Do You Have? (Model-to-Head Lookup)

The replacement procedure depends entirely on your trimmer head type, not just your model number. Confirm your head before following any steps below. The head type is usually stamped or molded on the cap of the trimmer head itself.

AutoCut C 25-2 vs. AutoCut C 5-2: Both are bump-feed, dual-exit heads. The C 25-2 accepts heavier line (up to 0.095″) and is rated for more demanding cutting. The C 5-2 is lighter-duty, paired with entry-level models, and typically limited to 0.065″–0.080″ line. The winding procedure is identical; only the line diameter and spool size differ.

What Line Diameter Does My Stihl Weed Eater Need?

Using the wrong diameter is the single most common cause of head jams, premature line breakage, and motor strain. Check the label molded into the trimmer head housing or your owner’s manual first. The table below covers standard factory specifications:

• Oversized line creates excessive drag on the head, causes the bump-feed mechanism to jam, and puts strain on the motor or engine.
• Undersized line breaks prematurely on contact with hard surfaces and fails to cut efficiently through dense vegetation.
• Line shape: Round line works in all Stihl AutoCut heads. Twisted and serrated profiles are compatible with most Stihl heads but can wear exit eyelets faster. Confirm compatibility on the head housing label before using non-round profiles.
• Length per strand: For AutoCut C 25-2 and C 26-2 heads, cut each strand to approximately 9 feet (about 2.7 m). For the C 5-2, cut to approximately 7–8 feet. Check the spool label — Stihl prints the maximum line length directly on most spool housings.
• Brand compatibility: Any brand of bulk trimmer line in the correct diameter works in Stihl heads. Stihl-branded line is not required, though Stihl’s own line is optimized for their exit eyelet geometry.

Safety and Tool Prep Before You Start

Complete these steps every time — skipping them is a hands injury risk.

• Gas models: Switch the engine off, set the throttle to the idle or stop position, and pull the spark plug wire off the spark plug. This prevents accidental ignition while your hands are near the cutting head.
• Battery models (FSA series, KombiSystem battery): Power off the unit and remove the battery pack before touching the head.
• Allow cooling: A recently run engine and head can be hot enough to cause burns. Wait at least 5 minutes.
• PPE: Wear cut-resistant gloves and safety glasses. Trimmer line under winding tension can snap back with significant force, and old line ends are sharp.
• Tools required: None in most cases — Stihl AutoCut and EasySpool heads are tool-free by design. The manual-wind fixed-line head on brush cutters requires an Allen key (hex key); the correct size is specified in your model’s operator manual.
• Leave the blade guard in place throughout the process.

How to Replace String on a Stihl AutoCut Dual-Exit Head (Most Common Method)

This procedure covers the AutoCut C 5-2, C 25-2, and C 26-2 heads — the stock heads on the majority of Stihl FS-series gas trimmers sold for residential and commercial use.

1. Open the head cap. On most AutoCut heads, press the two side tabs simultaneously and lift the cap straight off. On some older models, twist the cap counterclockwise (left) while holding the head housing. If the cap is stuck, tap the tabs gently with a rubber mallet — never pry with a screwdriver, which cracks the plastic tabs.
2. Remove the spool. Lift the spool straight out of the housing. Note the orientation — the arrow on the spool indicates the winding direction and must face up when you reinstall.
3. Remove all old line. Pull both strands of remaining line completely off the spool. Do not leave any old line on the spool; crossed or doubled strands are the primary cause of bump-feed jams after restringing.
4. Clean the spool and housing. Remove grass clippings, dirt, and debris from inside the head housing and off the spool itself. A stiff brush or compressed air works well. Check the bump-feed button spring — if it is missing or deformed, replace it before reassembling.
5. Cut two equal lengths of line. Use the diameter and length specifications from the section above. Cut cleanly with scissors or wire cutters; a jagged end is harder to thread through the eyelets.
6. Insert each strand into the spool’s center anchor eyelet. The spool has a center divider with one or two small holes or a molded anchor post. Insert one end of each strand into the corresponding anchor point. On dual-anchor spools, each strand has its own dedicated hole — do not thread both strands through one hole.
7. Wind each strand in the direction of the arrow. Wind both strands simultaneously, keeping each on its own half of the spool (above and below the center divider). Maintain tension as you wind — loose winding causes line to cross over itself inside the head and fuse or jam during use. Wind until approximately 6 inches (15 cm) of line remain on each strand.
8. Seat line ends in the retaining notches. The spool has two small notches or clips at the outer rim. Hook the end of each strand into a notch to hold tension while you reassemble. If the line pops free of the notches, the spool will unwind during reinstallation.
9. Thread both free ends through the exit holes. Hold the spool so the arrow faces the correct direction (match the arrow on the housing if visible), then feed each line end through its corresponding exit hole in the head housing. Pull each end through until the line releases from the retaining notches and seats fully on the spool.
10. Reinstall the spool and cap. Press the spool into the housing, align the cap tabs with the housing slots, and press down firmly until you hear or feel a click. On twist-lock caps, turn clockwise until the cap locks. Tug both line ends to confirm they feed freely.
11. Reconnect the spark plug wire (or reinsert the battery pack). Test-run the trimmer at low throttle for a few seconds to confirm the head spins correctly and the bump-feed advances line when the button taps the ground.

How much line should stick out after restringing? Most AutoCut heads have a built-in line cutter on the blade guard that trims line to the correct working length (typically 4–5 inches per side) on first use. Pull about 4–6 inches out before reassembly — the guard will trim the excess automatically.

How to Replace String on a Stihl EasySpool Head

EasySpool heads are found on newer FSA battery models and some KombiSystem attachments. The defining feature: no manual winding is required.

1. Power off and remove the battery pack.
2. Cut two lengths of 0.080″ line (do not exceed 0.080″ diameter — oversized line will not feed into the EasySpool’s self-winding slots and will damage the insert mechanism). Each length should be approximately 7–8 feet.
3. Insert one end of each strand straight into the two insertion slots on the head (slots are labeled or color-coded on most EasySpool versions). Push each strand in until it stops — approximately 1–2 inches of insertion depth.
4. Pull both strands outward firmly. The internal mechanism grips the line and winds it onto the internal spool automatically when you pull.
5. Leave approximately 4–5 inches of line extending from each exit hole.
6. Reinstall the battery pack and trigger the throttle briefly. The first rotation completes the winding tension and seats the line fully.

Common EasySpool mistake: Attempting to pre-wind line onto the spool manually before insertion. EasySpool’s mechanism is designed for straight-insertion loading only. Pre-winding jams the internal grip and prevents the self-winding function from engaging.

How to Swap a Pre-Wound Replacement Spool (Fastest Alternative)

If you are short on time or prefer not to wind line manually, Stihl and compatible aftermarket suppliers sell pre-wound replacement spools that drop directly into AutoCut heads. This is a separate task from rewinding — the spool is discarded and replaced entirely.

1. Open the trimmer head cap and remove the empty or depleted spool following steps 1–3 above.
2. Discard the old spool (or save it for rewinding with bulk line later — spools are reusable until the plastic cracks).
3. Drop the pre-wound replacement spool into the housing in the correct orientation (arrow up, matching the housing arrow).
4. Thread both line ends through the exit holes in the head housing.
5. Replace the cap, reconnect the spark plug wire or battery, and test-run.

Cost trade-off: Pre-wound spools cost roughly 3–5× more per use than the equivalent length of bulk line. For high-frequency users, rewinding with bulk line is substantially cheaper long-term. Pre-wound spools are model-specific — confirm the spool part number matches your exact head model before purchasing. Stihl lists compatible part numbers on their product pages and in the parts catalog for each trimmer model.

Are pre-wound spools universal? No. Spool diameter, core width, and line diameter all vary by head model. A spool for an AutoCut C 25-2 is not interchangeable with a C 5-2 spool.

How to Reload a Manual-Wind Fixed-Line Stihl Head

Brush-cutter models such as the FS 250 and FS 350 use a fixed-line head where the line is secured by a central bolt, not a spool housing. The bump-feed mechanism does not exist on these heads — line is manually advanced by stopping the engine and pulling it.

1. Power off, disconnect the spark plug wire, and allow the engine to cool completely.
2. Hold the head stationary using a block of wood or the trimmer’s blade-stop tool (if included). Loosen the center Allen bolt counterclockwise. The correct Allen key size is listed in your model’s operator manual — verify before applying force to avoid stripping the bolt head.
3. Remove the line stub completely. Note the guide hole or channel through which the line passes — this is a through-hole in the center post.
4. Cut a single length of the correct-diameter line (FS 250 and FS 350 typically take 0.105″–0.130″ line; confirm in your operator manual).
5. Insert the new line through the guide hole so that equal lengths extend from both sides of the head. Fold and center the line so both strands are equal.
6. Retighten the center Allen bolt to the torque specification in your model’s workshop manual. Do not over-tighten — the bolt secures the line by clamping, and excess torque cracks the head casting or strips the bolt threads. If you do not have a torque wrench, tighten firmly by hand plus approximately a quarter turn.
7. Pull both line ends to confirm they are clamped securely and do not slip when pulled hard.
8. Reconnect the spark plug wire and test at low throttle before full cutting use.

Troubleshooting: Common Stihl String Replacement Problems

Line feeds out continuously and won’t stop

Cause: Line ends were not seated in the spool’s retaining notches before the cap was installed, allowing the spool to unwind freely inside the head. Fix: Open the head, re-wind the line, and seat both ends firmly in the retaining notches before threading through the exit holes. Confirm the notches are not cracked or worn — a broken notch requires spool replacement.

Line breaks immediately on first use

Cause 1: Line diameter is too thin for the head or cutting conditions — the line cannot withstand the rotational force or impact with hard surfaces. Use the specified diameter for your model. Cause 2: The trimmer head is spinning in the wrong direction, causing the line to contact the blade guard at a stress angle. Confirm the head rotation direction matches the arrow on the guard.

Spool won’t seat in the housing

Cause: Debris is trapped in the housing bottom, or the spool tabs are not aligned with the housing slots. Fix: Clear all debris, confirm the bump-feed spring is seated correctly at the base of the housing, then align the spool tabs with the housing slots before pressing down.

Bump feed is not advancing line

Cause 1: Line crossed over itself during winding, preventing the spool from rotating inside the housing to advance line. Fix: Open the head and rewind, keeping each strand strictly on its own half of the spool. Cause 2: Line fusion — heat from extended use has melted the outer wraps of line together on the spool. Fix: Open the head and physically break the fused section apart, or replace the spool. Using quality line with a higher melt point reduces recurrence.

Head cap won’t open or won’t lock shut

Cause: Counterclockwise vs. clockwise confusion. Some Stihl models use a twist-off cap that turns counterclockwise to open; others use a press-tab release. Forcing the cap in the wrong direction strips the threads or snaps the tabs. Fix: Check your operator manual for the correct opening direction for your specific head. If the cap is genuinely stuck from debris, soak the head in warm water for 5 minutes to soften packed-in material before attempting to open.

Line tangled inside the spool

Cause: Both strands were wound on the same half of the spool, causing them to cross and bind. Fix: Remove all line from the spool, clean the spool, and rewind from scratch, keeping each strand strictly on its designated half (above and below the center divider). Wind both strands simultaneously in the same direction (per the arrow) to keep tension equal.

Frequently Asked Questions

Do I need to remove the spark plug wire every time I change Stihl weed eater string?

Yes, on gas models. Disconnecting the spark plug wire prevents accidental engine ignition while your hands are near the trimmer head. On battery models, remove the battery pack. This step takes under 10 seconds and is non-negotiable — a rotating trimmer head can cause severe hand injuries.

Which direction do I wind the string on a Stihl spool?

Wind in the direction indicated by the arrow molded on the spool itself. On most Stihl heads, this is counterclockwise when viewed from the bottom of the spool (the side facing the ground during use), because the head rotates clockwise during operation — winding against the rotation direction keeps tension on the line during cutting. Always follow the spool’s printed arrow, not a general rule, as head rotation direction varies by model.

Can I use any brand of trimmer line in a Stihl head?

Any brand of bulk trimmer line in the correct diameter works in Stihl AutoCut and EasySpool heads. Stihl-branded line is not required. Prioritize matching the diameter specification printed on the head housing over brand. Non-round line profiles (twisted, serrated) are compatible with most Stihl heads but confirm the head housing label does not restrict line profile before use.

Why does my Stihl trimmer string keep breaking after I just replaced it?

The three most common causes are: (1) line diameter is too small for the model — upgrade to the specified diameter, (2) line was wound loosely and slips inside the head, causing it to exit at an incorrect angle and contact the guard at a stress point — rewind with firm, even tension, or (3) the exit eyelets inside the head housing are worn or cracked, creating a sharp edge that cuts the line as it feeds out — inspect the eyelets and replace the head housing if the eyelets show notching or cracking.