How Do You Replace String on a Weed Wacker? (Step-by-Step Guide)

To replace string on a weed wacker, disconnect the power source, identify your trimmer head type (bump-feed, auto-feed, or fixed-line), remove the spool or head cover, extract the old string, and wind new string of the correct gauge onto the spool following the directional arrows. Leave 4–6 inches of line protruding through the eyelets, reinstall the spool, and test the tension by gently pulling the string—it should retract smoothly without binding or excessive resistance.

Understanding Your Weed Wacker String Head

Before you begin replacing string, you need to identify which type of trimmer head your weed wacker uses. Different head designs require slightly different replacement procedures, and using the wrong method can damage your equipment or result in poor string feeding.

Bump-Feed Trimmer Heads are the most common type found on residential trimmers. With this design, you manually bump the head against the ground while the trimmer is running, and the impact triggers a mechanism that feeds more string automatically. The spool is typically accessible by unscrewing a cover or releasing a snap-lock mechanism on top of the cutting head.

Auto-Feed Trimmer Heads automatically advance the string as it wears down during operation. These heads have a spring-loaded mechanism inside and require careful installation to ensure the feeding mechanism isn’t damaged. The spool design is often more complex than bump-feed models.

Fixed-Line Trimmer Heads have a stationary spool where the string length is predetermined and doesn’t automatically advance. You manually cut the string to the correct length and secure it, then replace it when worn. These are less common on modern consumer trimmers but still appear on some commercial and specialty models.

Check your trimmer’s manual or look for labeling on the head itself to determine which type you have. If you’ve lost the manual, search online for your specific trimmer brand and model number—manufacturer websites typically provide replacement guides and part diagrams.

String gauge matters too. Most residential trimmers use 0.065-inch or 0.080-inch diameter string, while heavier-duty models may require 0.095-inch or larger. Using the wrong gauge can cause feeding problems or reduce cutting efficiency. Your trimmer’s head or manual will specify the correct gauge.

Always prioritize safety when working on any power tool, including string trimmers.

• Disconnect Power Completely: If your trimmer is gas-powered, locate the spark plug wire and disconnect it by twisting and pulling the boot (the rubber connector) away from the spark plug. For cordless electric models, remove the battery completely. For corded electric trimmers, unplug from the outlet. Never work on a trimmer that could accidentally start during the replacement process.
• Allow the Engine to Cool: If you’ve been using the trimmer recently, wait at least 10–15 minutes for the engine to cool down. This prevents burns and reduces vapor risks from fuel.
• Work on a Clean, Flat Surface: Place the trimmer on a workbench or flat ground where you can see all parts clearly and prevent it from rolling or tipping.
• Wear Protective Gloves: Cut string ends and spool edges can be sharp. Wear work gloves to protect your hands, and consider safety glasses in case a spring or part suddenly releases tension.
• Keep Children and Pets Away: Ensure no one else is present during the replacement process, as small parts can pose choking hazards.

With power safely disconnected, you’re ready to access the cutting head.

Locate the Head Cover or Spool Cap: Look at the top of your trimmer’s cutting head. Most bump-feed and auto-feed models have a removable cover or cap. Some have a large knob or button you can unscrew by hand (usually counterclockwise). Others use a snap-lock design where you press tabs or squeeze a clip to release the cover. If you’re unsure, consult your manual or search for your model online—a quick video guide can show you exactly which direction to turn.

Remove the Spool: Once the cover is off, the spool (the bobbin that holds the wound string) should lift out easily, sometimes with a spring beneath it. If the spool doesn’t come free, check for a center screw or bolt holding it in place. Some models require a socket wrench or screwdriver to remove the center fastener first.

Extract Old String: Pull away any remaining string from the spool and the eyelets (small guides on the side of the head where string feeds through). Remove any tangled or broken bits completely. If string is heavily wound and stuck, gently pry it away with a plastic spoon or old credit card—avoid metal tools that might scratch the spool and cause future feeding problems.

Inspect the Spool and Head: Take a moment to examine the spool for cracks, warping, or damage. Check the eyelets for rough spots or cracks as well. If the spool is damaged, it needs replacement (most are inexpensive). Similarly, if an eyelet is broken, the entire head assembly may need replacement, though this is rare.

Determine String Length: Most trimmers require 15–25 feet of new string per spool, depending on the model. Your manual will specify the exact length needed. If you don’t have the manual, a common rule of thumb is to fill the spool until the string is level with the outer rim, leaving room for the cover to close without crushing it. When in doubt, start with 20 feet and adjust based on how tightly you wind.

Locate the String Slot or Anchor Point: Most spools have a small notch, slot, or hole near the center where you secure the end of the string before winding. Some models have two slots (one for each string segment on dual-line heads). Look for labeling or arrows indicating where to start.

Secure the String End: Insert one end of the new string into the anchor slot. Push it in firmly until it catches or locks in place. If there’s no audible click, it should still be held snugly by friction.

Wind the String Tightly: Wind the string onto the spool in the direction indicated by arrows on the spool itself. This is critical—winding in the wrong direction will cause the string to unravel during operation. Wind in smooth, even layers, working from one side of the spool toward the center, then back to the other side. Keep tension on the string as you wind to ensure it’s packed tightly. A loose wind will cause uneven feeding and bunching.

Leave String Protruding: As you near the end of the string length, stop winding and leave 4–6 inches of string free (not wound on the spool). This extra length is critical—it threads through the eyelets and provides the actual cutting line. If you wind it all onto the spool, you’ll have no line to cut with.

Thread Through the Eyelets: Most trimmers have two eyelets (guide holes) on opposite sides of the cutting head. Thread one end of the remaining free string through one eyelet and the other end through the other eyelet. Pull gently to ensure the string seats properly in each guide.

Dual-Line vs. Single-Line Heads: If your trimmer uses two separate pieces of string (dual-line), repeat the winding and threading process for a second string segment. Some heads have a single continuous line looped around the spool instead—in that case, wind both ends together as a loop, then thread each end through its eyelet.

Reinstall the Spool: Carefully lower the loaded spool back into the head assembly, aligning any tabs or guides with their slots. If there was a spring under the spool, ensure it’s positioned correctly before reinstalling the spool—a misaligned spring can interfere with the feeding mechanism.

Reattach the Head Cover: Place the cover or cap back on top and secure it. If it screws on, turn it clockwise firmly but not with excessive force—over-tightening can crack the cover or strip the threads. If it’s a snap-lock design, press or click it until it seats fully.

Verify String Position: Before reconnecting power, visually confirm that the string ends are protruding evenly through both eyelets on each side. They should be approximately the same length. If one side is much shorter, the string may not have threaded correctly.

Test Tension: Reconnect your power source (battery, spark plug wire, or electrical outlet) and hold the trimmer firmly. Do not start the engine yet. Gently pull on the string by hand—it should have moderate resistance and retract smoothly when you release it. If it’s extremely tight and won’t pull, or if it’s slack and hangs limply, the tension is incorrect. In that case, disconnect power again, remove the cover, and adjust by rewinding more tightly or loosely as needed.

Test Operation (Briefly): Once you’re confident the installation is correct, start the trimmer in an open area away from people and objects. Let it run for just 10–15 seconds at idle speed. Listen for unusual sounds and watch the string to ensure it’s spinning smoothly and symmetrically. Stop and inspect if anything seems off.

String Keeps Tangling After Replacement: This usually means the string wasn’t wound tightly enough or in the wrong direction. Remove the cover, unwind the string completely, and start over, ensuring you wind in the direction of the arrows and keep consistent tension throughout. Also check that the string ends are threaded straight through the eyelets without twisting.

String Won’t Feed or Dispense: For bump-feed models, the string may not be advancing because the bump mechanism is damaged or the string is pinched. Inspect the bump pad under the head for wear or cracks. If it’s damaged, it needs replacement. If the pad looks fine, the string may be wound too tightly or pinched between the spool and cover. Try loosening slightly and reinstalling. For auto-feed models, a feeding problem often indicates the internal spring mechanism is stuck or the string is in the wrong slot. Consult your manual for your specific model’s auto-feed design.

String Is Too Loose or Too Tight: Loose string results in poor cutting power and uneven line wear. Tight string creates excessive resistance when you try to pull it and can cause the motor to work harder than necessary. If adjusting the spool’s position in the head doesn’t help, you may need to rewind the string with adjusted tension. Generally, aim for firm but not rock-hard resistance when you manually pull the string.

Trimmer Not Cutting Evenly After Replacement: Uneven cutting usually indicates the two string ends are different lengths. Measure them and trim the longer one to match the shorter. Also check that both ends are properly seated in their eyelets—a string that’s partially out of an eyelet won’t cut properly on that side.

String Breaks Immediately During Use: This can happen if the new string is hitting something or if the eyelets have rough edges or burrs. Inspect the eyelets closely and gently smooth any rough spots with fine-grit sandpaper. Also ensure nothing inside the head is blocking the string path. If the problem persists, the head assembly may be damaged and need replacement.

Replace String Regularly: For homeowners who use their trimmer weekly during the growing season, plan to replace string every 3–6 months. Commercial or heavy-use trimmers may need replacement monthly or even more frequently. Inspect your string visually—if it’s visibly frayed, thinned, or shorter than it should be, it’s time for replacement.

Avoid Hitting Hard Objects: Striking rocks, concrete, or metal fences quickly wears string and can break it immediately. When trimming near driveways, sidewalks, or decorative borders, work carefully and slowly. String replacement will be less frequent if you maintain good cutting technique and avoid obstacles.

Store Correctly: When not in use, store your trimmer in a dry location away from direct sunlight. UV light degrades string over time, especially if the trimmer is left outside or in a shed with significant sun exposure. If you store the trimmer for an extended period (winter months), consider removing the string to prevent it from becoming brittle or warped.

Keep the Head Clean: After each use, wipe down the cutting head with a dry cloth to remove grass clippings and debris. This prevents buildup that can interfere with the feeding mechanism or damage the string. A clean head also makes your next string replacement easier.

Use the Correct String Gauge: Always replace with the gauge specified in your manual. Using heavier string than recommended can strain the motor and feeding mechanism. Using lighter string may not cut thick weeds effectively. Sticking to the manufacturer’s specification ensures optimal performance and longevity.

Q: How do I know what string gauge I need?
A: Check your trimmer’s manual, the packaging the trimmer came in, or the manufacturer’s website for your specific model. Common gauges are 0.065 inch, 0.080 inch, and 0.095 inch. Using the wrong gauge can cause feeding problems and poor cutting. When in doubt, contact the manufacturer or visit a local equipment dealer who can verify your trimmer’s specifications.

Q: Can I use any brand of replacement string, or do I need the manufacturer’s brand?
A: You don’t need to use the original manufacturer’s string. Generic aftermarket string of the correct gauge works just as well and is often less expensive. However, ensure it’s the right diameter—this is more important than the brand. Buying from reputable outdoor equipment retailers ensures you’re getting quality string that meets specifications.

Q: What’s the difference between single-line and dual-line trimmer heads?
A: Single-line heads have one continuous piece of string looped around the spool, while dual-line heads have two separate pieces of string wound on the spool. Dual-line heads cut a wider swath and are more efficient for larger areas, but they also use string faster. Single-line heads are simpler and better for small yards or detail work. The replacement process is nearly identical—just wind two sections instead of one if using a dual-line head.

Q: Why does my new string keep breaking right after I replace it?
A: Breaking immediately usually indicates the string is hitting something inside the head or an eyelet has a rough edge or burr. Remove the head cover, inspect the eyelets closely, and smooth any rough spots with fine-grit sandpaper if needed. Also check that nothing is blocking the string path. If the problem continues after inspection, the head assembly may be damaged and require replacement.

Q: How tight should I wind the string on the spool?
A: Wind tightly enough that the string doesn’t shift or slip when you gently tug on it, but not so tight that you need significant force to wind it. The string should be packed in neat, even layers without gaps or overlaps. A good test is whether you can manually pull the installed string with moderate resistance—not too easily, but not with extreme force either.