How Do You Replace String on a Stihl Weed Eater? (Step-by-Step Guide for Any Head Type)

Identify Your Stihl Head Type Before You Start

Stihl weed eaters ship with several different trimmer head designs. Using the wrong replacement method damages the head or wastes time. Visually inspect the head while the trimmer is off to determine your path before touching any hardware.

The Three Main Stihl Head Types

• Bump-Feed AutoCut Head: The most common Stihl head. Identified by a large plastic cap on the bottom that you tap against the ground to advance the line. Line is wound on an internal spool. AutoCut heads include models such as AutoCut 25-2, AutoCut C 5-2, AutoCut 46-2, and AutoCut 36-2. If you see a round bump knob on the underside and two small line exits (eyelets) on the sides, this is your head type.
• Fixed-Line PolyCut Head: Identified by a hard plastic star-shaped or disc-shaped head with no bump knob and no internal spool. Line segments are pre-cut and inserted directly into slots or channels around the perimeter. PolyCut 20-3, PolyCut 41-3, and PolyCut 6-3 are common examples. These heads do not require winding.
• Blade Head: Uses plastic or metal blades rather than flexible line. No string replacement is needed — blades are swapped individually. This guide does not cover blade heads.

If you cannot locate a bump knob on the underside and cannot see line eyelets on the sides, check your trimmer’s model number (stamped on the engine housing) against Stihl’s head compatibility list before proceeding.

Choose the Right Trimmer Line Diameter for Your Stihl Model

Installing the wrong diameter line is the second most common restringing mistake. Line that is too thick will not feed through the eyelets. Line that is too thin snaps immediately under load. Match diameter to your specific model series using the reference table below.

Line material: Round nylon monofilament is sufficient for grass and light weeds. Copolymer or twisted/serrated line cuts heavier brush more aggressively but wears faster. Stihl’s own branded trimmer line (available in .065, .080, .095, and .105 in) is formulated for AutoCut spool tension and is a reliable baseline choice. Third-party round nylon in the correct diameter also performs acceptably.

How much line to prepare: Most Stihl residential spools (AutoCut 25-2, C 5-2) hold approximately 20–25 feet (6–7.5 m) of line per strand. Professional heads (AutoCut 46-2) can hold up to 30 feet (9 m) per strand. Cut two equal lengths for dual-line spools or one length for single-line spools before starting assembly.

Safety Prep and Tools You Need

Complete these steps before any disassembly. Skipping them creates real injury risk from accidental engine start or sharp spent-line edges.

1. Turn off the engine and allow the trimmer head to come to a complete stop.
2. Disconnect the spark plug wire. Pull the rubber boot off the spark plug tip. This prevents any accidental ignition while your hands are near the head. Stihl operator manuals list this as a mandatory safety step — not advisory — for any service or adjustment work near the cutting attachment.
3. Put on work gloves. Spent trimmer line has sharp, frayed ends. New line is also stiff and can cut skin under tension.
4. Set the trimmer on a stable flat surface with the head accessible. A workbench or a picnic table works well.

Tools and materials checklist:

• Replacement trimmer line in the correct diameter (see table above)
• Work gloves
• Flathead screwdriver or coin (some AutoCut cap variants require a twist-off rather than tab press)
• Tape measure or pre-measured line lengths
• Clean rag (to wipe grass debris from head before reassembly)

How to Replace String on a Stihl Bump-Feed (AutoCut) Head

Follow these steps in order. Do not skip the winding-direction step — winding backward is the single most common cause of line-feed failure after restringing.

Step 1 — Remove the Bump Cap

Most Stihl AutoCut heads use a press-tab release. Grip the head body firmly with one hand. With the other hand, locate the two tabs on opposite sides of the bump cap. Press both tabs inward simultaneously and pull the cap straight down. Some older heads (AutoCut 25-2 on FS 80/85 series) use a left-hand thread cap — turn clockwise (reverse of normal) to loosen. If the cap does not move with tab pressure, try inserting a flathead screwdriver into the tab slots for additional leverage.

Step 2 — Extract the Spool

With the cap off, lift the spool straight out of the head housing. Note the spring beneath it — do not lose this spring. It sits between the spool and the housing and must be reinstalled in the same orientation. Set the cap and spring aside on a clean surface.

Step 3 — Remove Old Line

Unwind and discard all remaining old line from the spool. Check the spool body for cracks, worn line channels, or melted plastic. A cracked spool will cause immediate line-feed failure even with new line installed — replace the spool (available at Stihl dealers by head model number) before proceeding.

Step 4 — Prepare the New Line

For a dual-line spool (two exit eyelets on the head — most common): cut two equal lengths of line, approximately 20 feet (6 m) each, or the length specified on the spool label.
For a single-line spool (one exit eyelet): cut one length of approximately 20–25 feet (6–7.5 m).

Locate the small anchor hole or notch in the center divider of the spool. On dual-line spools there will be two anchor points, one per winding channel.

Step 5 — Wind Line in the Correct Direction

This step determines whether the line feeds properly during use. Find the arrow molded into the spool body. This arrow shows the required winding direction. Winding against this arrow causes the line to advance continuously or not at all during bump-feed operation, because the internal cam mechanism relies on the line tension direction matching the spool’s rotation direction.

1. Insert one end of the line into the anchor notch in the spool’s center.
2. Wind the line tightly and evenly in the direction the arrow indicates, keeping rows flat and not crossing lines over each other.
3. Leave approximately 6 inches (15 cm) of line unwound at the end. Secure this tail in one of the small holding notches on the outer spool rim — these notches hold the line in place during reassembly and are released automatically when the spool is seated.
4. Repeat for the second channel if using a dual-line spool.

Step 6 — Thread Line Through the Eyelets

Hold the head housing in one hand and the loaded spool in the other. Feed each line tail from the spool’s holding notch through the corresponding eyelet on the head housing. The eyelets are the small metal- or plastic-lined holes on the sides of the head body. Pull each tail through until the spool seats fully into the housing with no slack between the spool and the eyelet exit point.

Step 7 — Reinstall the Spring and Snap the Cap On

Drop the spring back into the spool center (or housing base — placement varies by AutoCut model; check that it sits flat before proceeding). Align the bump cap’s tabs with the housing slots and press firmly upward until both tabs click into place. Tug the cap — it should not move. Pull each line tail to confirm it moves freely through the eyelet without binding.

Step 8 — Test the Bump-Feed

Reconnect the spark plug wire. Hold the trimmer at operating angle, start the engine, allow the head to reach full speed, then tap the bump knob firmly against a patch of bare ground. The line should advance 1–2 inches (2.5–5 cm) per tap. If the line does not advance, see the Troubleshooting section below.

How to Replace String on a Stihl Fixed-Line (PolyCut) Head

PolyCut heads use pre-cut line segments rather than a wound spool. Replacement requires no winding and no special tools.

Step 1 — Identify Slots and Remove Spent Segments

With the engine off and spark plug wire disconnected, examine the PolyCut head. Segments are held in radial slots around the head perimeter. Grip each spent segment and pull it straight outward from its slot. Some segments will be worn to stubs — grip with pliers if needed. Remove all spent segments before installing new ones.

Step 2 — Select and Cut Replacement Segments

PolyCut heads are designed for use with Stihl’s own pre-packaged segment sets, which are sized to the specific head model. The PolyCut 20-3 uses segments approximately 12 cm (4.7 in) long; the PolyCut 41-3 uses segments approximately 20 cm (7.9 in) long. Verify the segment length on the head’s label or the packaging. Do not substitute bulk spool line cut to length — the segment material and stiffness differ from wound line and are part of the PolyCut head’s cutting mechanics.

Step 3 — Insert New Segments

Push each new segment into its slot, folded at the center if the head uses a fold-through design, until it clicks or seats flush with the slot entry. Tug each segment to confirm it is locked. A loose segment will eject at speed and become a projectile hazard.

Step 4 — Verify and Restart

Reconnect the spark plug wire. Start the trimmer and allow the head to reach full speed in an open area away from bystanders. PolyCut heads do not require a bump-feed test — line is fixed and does not advance. Inspect for even line length around all segments after the first use pass.

Troubleshooting Common Problems After Restringing

Line Does Not Feed When Head Is Tapped

• Line wound in wrong direction: Remove the spool and rewind in the direction of the arrow. This is the most common cause.
• Line crossed or overlapping on spool: Crossed windings create friction that prevents the spool from rotating inside the head. Rewind with flat, even rows.
• Spring missing or inverted: Without the return spring, the spool cannot advance. Reinstall the spring in correct orientation.
• Eyelets worn or cracked: A rough eyelet edge grips the line. Inspect the metal eyelet inserts — replace the head if the eyelets are damaged.

Line Melts or Fuses Together on the Spool (Line Welding)

Fusing happens when the spool overheats during heavy use, causing adjacent windings to bond. Prevent it by: winding line tightly with no loose loops, keeping winding rows flat, and not operating the trimmer at full throttle continuously for extended periods without breaks. If fusing occurs, remove the spool, cut away the fused section, and rewind with the remaining line or fresh line.

Line Feeds Too Much / Keeps Advancing Without Tapping

• Bump cap is not fully seated: The cap’s inner cam must lock against the spool. Press the cap firmly until both tabs click. A cap that is slightly off-center allows the spool to advance freely from vibration alone.
• Worn bump knob: The plastic bump surface wears down over time. Replace the cap assembly if the knob surface is flat or cracked.

Line Breaks Immediately After Installation

• Wrong diameter for the model: Undersize line lacks the tensile strength for the head’s RPM. Verify diameter against the table in this guide.
• Old or brittle line: Nylon trimmer line dries out and becomes brittle over time. Store unused line in a sealed bag or soak new line in water for 30 minutes before winding to restore flexibility.
• Head spinning too close to hard surfaces: Line breaks instantly when it contacts fencing, concrete edging, or gravel. Maintain a safe distance from hard obstacles.

Head Does Not Spin Freely After Reassembly

Line debris or grass cuttings packed inside the head housing create drag. Before reinstalling the spool, wipe the inside of the housing clean with a rag. Check that the spool rotates freely by hand before snapping the cap on.

Frequently Asked Questions

Is it mandatory to disconnect the spark plug wire before changing Stihl trimmer line?

Yes. Stihl operator manuals for all FS-series trimmers list spark plug wire disconnection as a required safety step before any work on the cutting attachment. An engine that fires while your fingers are in contact with the head can drive the rotating spool cap into your hand. The procedure takes under five seconds and eliminates that risk entirely.

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

Any round nylon monofilament line in the correct diameter for your model will work. The critical variable is diameter, not brand. Avoid flat, star-shaped, or heavily textured specialty lines in AutoCut heads — their geometry can bind in the eyelets or create uneven spool tension that prevents proper bump-feed advancement. PolyCut heads require Stihl’s own pre-cut segment sets; bulk spool line is not a compatible substitute.

How often should I replace the trimmer line on my Stihl weed eater?

There is no fixed interval. Replace the line when it is completely consumed from the spool, when the remaining length is too short to exit the eyelets by at least 4–5 inches (10–12 cm), or when visible cracking and brittleness appear on the line even before it is consumed. Heavy use on rough or stony ground consumes line significantly faster than trimming soft grass.

What is the difference between a single-line and a dual-line Stihl spool?

A single-line spool has one winding channel and one exit eyelet on the head — the line runs from a single wound strand. A dual-line spool has two winding channels separated by a center divider and two exit eyelets — each channel holds a separate strand. Dual-line configuration cuts more aggressively and is standard on mid-range and commercial Stihl models (FS 56 and above). The replacement process is the same for both; dual-line simply requires winding two separate strands, one per channel.