How Do You Install Weed Trimmer String? (Complete Guide for Every Head Type)

Quick Answer: How to Install Weed Trimmer String

Power off the trimmer completely (remove the battery, unplug the cord, or disconnect the spark plug wire on gas models), press the release tabs to remove the spool head, wind the correct-gauge line in the direction marked on the spool, thread the ends through the eyelets, and snap the head back into place. The exact method depends on your head type — bump-feed, fixed-line, and automatic-feed each use a different loading process, covered in detail below.

What Type of Trimmer Head Do You Have?

Identifying your head type before touching the spool prevents following the wrong steps entirely. The three common types look and behave differently:

• Bump-feed head: Has a rounded plastic knob on the bottom. You tap it against the ground while the trimmer runs to advance more line. Most common head type on residential and semi-commercial trimmers. The spool is internal and must be wound manually.
• Fixed-line head: Has no knob and no internal spool. Pre-cut line segments push directly into ports on the head body. The head is typically smaller and lighter than a bump-feed head.
• Automatic-feed head: Resembles a bump-feed head externally but advances line via an internal tensioning or centrifugal sensor — no tapping required. Most automatic heads still require manual spool winding; “automatic” describes how line advances, not how it loads.

The head type is usually printed on the cap label or listed in the owner’s manual under “cutting head” or “trimmer head.” If the label is worn off, check the trimmer’s model number on the manufacturer’s website.

How to Identify the Correct Trimmer Line Gauge

Using the wrong line diameter causes immediate problems: undersized line snaps on contact with dense weeds; oversized line jams the head and strains the motor. Find the required gauge before purchasing replacement string.

Where to find your required gauge:

• Printed on the spool cap (most common location)
• Stamped or molded into the trimmer head housing
• Listed in the owner’s manual under “replacement line” or “accessories”
• On the trimmer’s product page using the model number from the housing label

The three most common gauges by application:

Always match the gauge printed on your spool cap. Substituting a thinner gauge to “make it work” accelerates line breakage and increases cutting time. Substituting a thicker gauge risks locking the spool inside the head.

Safety First: How to Power Off Your Weed Trimmer Before Restringing

The head must be completely stopped and cool before handling. Steps differ by power source.

Gas Trimmer

1. Turn the engine off using the on/off switch.
2. Let the engine cool for at least 5 minutes.
3. Disconnect the spark plug wire by pulling the rubber boot off the spark plug tip. This prevents accidental starts while your hands are near the head.

Battery (Cordless) Trimmer

1. Press the power button to shut the trimmer off.
2. Remove the battery pack by pressing the release latch and sliding the pack out.
3. Set the battery aside — do not reinstall it until the head is fully reassembled.

Corded Electric Trimmer

1. Release the trigger and wait for the head to stop spinning completely.
2. Unplug the power cord from the outlet.
3. Do not rely on the trigger lock alone — unplug before touching the head.

How to Install String on a Bump-Feed Trimmer Head

Bump-feed heads are the most common type and the most involved to restring. They use either a single-line or dual-line spool — the winding method differs between the two.

Step 1: Remove the Bump Knob and Spool

1. Hold the trimmer head steady with one hand.
2. Turn the bump knob counterclockwise to unscrew it (some models require pressing side tabs while turning). Note: on some gas trimmers the knob threads reverse — check your manual if counterclockwise doesn’t release it.
3. Lift off the bump knob, then remove the spring (set it aside — losing the spring is the most common reassembly problem).
4. Lift the spool out of the head housing.
5. Remove any old line remnants from the spool.

Step 2: Measure the Correct Line Length

Cut the replacement line to length before winding. Spool capacity varies by head size — smaller residential heads hold roughly 9–15 ft per strand; larger semi-commercial heads hold 20–25 ft per strand. Check the spool cap or manual for the exact capacity for your model. When in doubt, cut to the maximum listed length — excess line is trimmed automatically when the head advances.

Step 3: Wind a Single-Line Spool

1. Find the small anchor hole or notch on the spool center post.
2. Fold one end of the line and insert it into the anchor hole about 1/2 inch, or tie a small overhand knot if the spool uses a notch anchor.
3. Locate the wind-direction arrow molded onto the spool body — wind in that direction only. Winding opposite to the arrow causes the line to unwind rather than advance when you bump the head.
4. Wind the line in tight, even rows without crossing. Crossed lines cause the spool to jam.
5. Stop winding when approximately 6 inches of line remain, and secure the end in one of the holding notches on the spool rim.

Step 4: Wind a Dual-Line Spool

Dual-line spools have two separate winding chambers divided by a center flange.

1. Cut two equal lengths of line.
2. Insert one end of the first strand into the anchor hole in the top chamber.
3. Wind the first strand in the direction marked on the spool, filling only the top chamber. Secure the tail in a rim notch.
4. Insert the second strand into the anchor hole in the bottom chamber.
5. Wind the second strand in the same rotational direction as the first, filling only the bottom chamber. Secure the tail in a rim notch.
6. Do not cross strands between chambers — this locks the spool.

Step 5: Thread the Line Through the Eyelets and Reassemble

1. Release the line ends from the rim notches.
2. Feed each line end through its corresponding eyelet (the small metal-lined hole in the outer head housing). On a single-line spool there is one eyelet; on a dual-line spool there are two, one on each side.
3. Pull 4–6 inches of line through each eyelet.
4. Drop the spool back into the head housing, pressing it down until it seats flush.
5. Replace the spring over the spool center post.
6. Press the bump knob down over the spring and thread it clockwise until snug. Do not overtighten — the knob needs to move freely to function.

How to Install String on a Fixed-Line (Pre-Cut) Trimmer Head

Fixed-line heads require no winding. Pre-cut segments insert directly into ports on the head. This is the fastest restringing method.

1. Check the required segment length in your owner’s manual or on the head housing — most fixed-line heads use segments between 6 and 9 inches long.
2. Cut the replacement line to the specified length. Both strands must be equal.
3. Locate the insertion ports on the head — typically two ports positioned 180 degrees apart.
4. Fold one segment in half (for heads using a single folded strand) or insert individual segments (for heads with two separate ports). Refer to the label on the head for the correct method.
5. Push the line into the port firmly until you feel or hear it click into the locking tab.
6. Pull gently on the line to confirm it is locked — it should not slide back out.
7. Confirm equal line length protruding from both sides. Unequal lengths cause vibration during operation.

No reassembly is needed — the head body does not open. If line won’t lock, confirm you are using the correct line diameter; undersized line does not trigger the locking tab.

How to Install String on an Automatic-Feed Trimmer Head

Automatic-feed heads advance line without bumping, but most still require manual spool winding. The loading process mirrors the bump-feed method above, with one key difference in reassembly.

1. Remove the head cap by pressing the side tabs and pulling it off — there is no bump knob to unscrew.
2. Remove the spool and clear old line.
3. Wind the new line following the direction arrow on the spool (same process as bump-feed, Steps 2–4 above).
4. Thread the line ends through the eyelets.
5. Reseat the spool and snap the cap back on. The cap is flush — it does not protrude like a bump knob.

Exception — pre-wound replacement spools: Some automatic-feed models (including certain Husqvarna and ECHO Speed-Feed variants) accept factory pre-wound drop-in spools. For these, skip winding entirely: remove the old spool, drop in the pre-wound replacement, thread the line ends through the eyelets, and snap the cap closed. Check your model’s accessory page to confirm whether pre-wound spools are available before purchasing bulk line.

Common Mistakes When Installing Trimmer String

• Winding in the wrong direction: The line won’t advance when you bump the head or trigger the auto-feed. Always follow the arrow on the spool body — it is always present and always correct.
• Line ends too short after threading: Leaving less than 4 inches through the eyelets means the head runs out of line within minutes of use. Pull 5–6 inches through each eyelet before reassembling.
• Line ends too long: Excessive line length (over 8–9 inches beyond the guard) causes the trimmer to drag, vibrate heavily, and strain the motor. The blade guard on the trimmer housing will trim excess line automatically on most models — let it do so on the first pass.
• Using the wrong gauge: Undersized line snaps immediately on heavy growth. Oversized line jams in the eyelets or locks the spool. Match the gauge on the spool cap.
• Spool not fully seated: If the bump knob rattles or the head wobbles in operation, the spool is not seated flush. Remove the knob and press the spool firmly down before reassembling.
• Crossed lines on a dual-line spool: One strand wound over the other in the same chamber causes the spool to bind. Keep each strand strictly within its own chamber.
• Forgetting the spring: Reassembling the bump knob without the spring produces a head that will not advance line and will rattle. If bump advance stops working immediately after restringing, disassemble and check for the spring.

How Often Should You Replace Weed Trimmer String?

Replacement frequency depends on usage intensity and the material being cut.

• Light residential use (edges, thin grass, 1–2 sessions per week): Reload every 3–4 mowing sessions or when visible fraying or shortening occurs.
• Moderate residential use (mixed weeds, larger yard areas): Inspect the line after every session. Replace when either strand is shorter than 4 inches or shows brittleness.
• Heavy use or commercial: Inspect before every use. Line may require replacement every 1–2 sessions when cutting dense brush or along hard surfaces (concrete, brick, gravel) that accelerate wear.

Signs the line needs replacing:

• Line is visibly shorter than 4 inches on one or both sides
• Cutting performance has dropped noticeably — the trimmer is pushing weeds rather than cutting them
• Line is brittle, cracked, or discolored from UV exposure (line stored in direct sunlight degrades faster)
• Frequent line breaks mid-session despite correct gauge

Storage note: Trimmer line does degrade over time in storage, particularly in hot or UV-exposed environments. Line stored for more than two years in a garage or shed may be brittle before first use. Store unused line in a sealed bag away from direct sunlight.

Frequently Asked Questions

Which direction do I wind trimmer line on a bump-feed spool?

Wind in the direction of the arrow molded on the spool body. The arrow is always present on the spool face and indicates the correct rotational direction for your specific head. Winding opposite to the arrow causes the line to unwind into the housing rather than advance through the eyelets when the head is bumped.

How much trimmer line do I need per spool?

Spool capacity varies by head size. Small residential bump-feed heads typically hold 9–15 ft per strand; larger heads hold up to 20–25 ft per strand. The maximum line length is usually printed on the spool cap or in the owner’s manual under “line capacity.” Cut to the listed maximum — the head trims excess on the first run.

Do I need to remove the spark plug before changing trimmer line on a gas trimmer?

Disconnect the spark plug wire (pull the rubber boot off the spark plug tip) before removing or restringing the head on any gas trimmer. This eliminates the risk of accidental engine start while your hands are at the cutting head. Turning the engine off alone is not sufficient — the engine can still fire if the pull cord is accidentally engaged.

Why does my trimmer line keep breaking immediately after I install it?

The three most common causes are: (1) using a line gauge thinner than the one specified for your model, which snaps on contact with dense vegetation; (2) winding the line with crossed strands that create weak pressure points; (3) using aged or UV-degraded line that is already brittle. Confirm the gauge matches the spool cap specification, rewind with even non-crossing rows, and replace stored line that is more than two years old.