Introduction
Restringing a Stihl FS 40C trimmer is a routine maintenance task that keeps your weed eater cutting cleanly and efficiently. Whether you are a homeowner tackling a weekend yard project or a landscaping professional who relies on this tool daily, knowing how to string a Stihl weed eater Fs40c correctly saves time, reduces frustration, and extends the life of the cutting head. This guide walks you through every stage of the process, from understanding the trimmer head design to selecting the right line, executing the restringing steps, and maintaining the tool afterward. By the end you will have a clear, repeatable procedure you can follow whenever the line wears down or breaks.
- Understanding the FS 40C Trimmer Head
- Tools and Materials Needed
- Safety Precautions
- Step‑by‑Step Guide to Stringing the FS 40C
- Choosing the Right Trimmer Line
- Trimmer Line Comparison Table
- Tips for Optimal Performance
- Common Mistakes and How to Avoid Them
- Maintenance After Restringing
- Troubleshooting Common Issues
- Frequently Asked Questions
- Conclusion
Understanding the FS 40C Trimmer Head
The Stihl FS 40C uses a fixed‑line, bump‑feed head that holds a single length of nylon line wound onto a spool. Unlike dual‑line heads, this design feeds line automatically when you tap the head against the ground, which means the line must be wound in the correct direction and with the proper tension. The head consists of a housing, a spool, a spring‑loaded bump knob, and a line‑exit eyelet. Familiarity with these components helps you avoid cross‑threading the line or damaging the spring mechanism during restringing. The spool is marked with an arrow indicating the winding direction; following that arrow is essential for reliable feed.
Tools and Materials Needed
Before you begin, gather the following items so the job proceeds without interruptions:
- Replacement trimmer line (typically .080‑inch or .095‑inch diameter for the FS 40C)
- Clean rag or shop towel
- Small flat‑head screwdriver (for prying the bump knob if needed)
- Protective gloves
- Safety glasses
- Owner’s manual (for reference to the specific line length recommendation)
Having the correct line diameter is critical; using a line that is too thick can jam the head, while a line that is too thin may wear out quickly and reduce cutting efficiency.
Safety Precautions
Always treat the trimmer as a powered cutting tool even when the engine is off. Follow these safety steps before you start:
- Disconnect the spark plug wire to prevent accidental starts.
- Wear safety glasses to protect your eyes from debris or a snapped line.
- Wear gloves to protect your hands from sharp edges on the spool and housing.
- Work on a stable, flat surface with good lighting.
- Keep bystanders, especially children and pets, at a safe distance.
These precautions are simple but they eliminate the most common injuries associated with line replacement.
Step‑by‑Step Guide to Stringing the FS 40C
Follow these ordered steps carefully. Each step includes a brief explanation of why it matters.
- Remove the bump knob. Turn the knob counter‑clockwise until it releases. This exposes the spool and spring.
- Take out the spool. Lift the spool straight out of the housing. Note the orientation of the arrow on the spool flange.
- Clean the head. Wipe any grass clippings, dirt, or old line fragments from the housing, spool, and spring with a rag. A clean head prevents debris from interfering with line feed.
- Cut the new line to length. Consult the manual; most FS 40C models require about 10‑12 feet (3‑3.5 m) of line. Cut a single piece to that length.
- Insert the line into the anchor hole. Feed one end of the line through the small anchor hole on the spool flange. Pull it through until roughly 2‑3 inches remain protruding.
- Wind the line following the arrow. Hold the spool steady and wind the line tightly and evenly in the direction indicated by the arrow. Keep each coil snug against the previous one; loose coils cause feeding problems.
- Leave a short tail. After winding the required amount, leave about 4‑6 inches of line free at the outer edge of the spool. This tail will be pulled through the eyelet later.
- Reinstall the spool. Place the spool back into the housing, aligning the anchor hole with the eyelet on the housing.
- Thread the tail through the eyelet. Pull the free tail through the eyelet until the line sits flush with the housing opening.
- Replace the bump knob. Screw the knob back on clockwise until it seats firmly. Do not overtighten; the knob should turn freely for bump feeding.
- Reconnect the spark plug wire. Restore the ignition circuit.
- Test the feed. Start the engine, let it warm up, then tap the head gently on the ground. The line should advance smoothly. If it does not, repeat steps 5‑9, checking for proper winding direction and tension.
Choosing the Right Trimmer Line
Line selection influences cutting speed, durability, and the types of vegetation you can tackle. The FS 40C is designed for medium‑duty residential use, so a .080‑inch round line works well for grass and light weeds. For thicker brush or occasional small saplings, a .095‑inch square or twisted line provides more cutting edge. Always match the line diameter to the specifications in the owner’s manual; using an oversized line can over‑stress the motor and cause premature wear on the head bearings.
Trimmer Line Comparison Table
| Line Type | Diameter | Best For | Durability | Cutting Aggressiveness |
|---|---|---|---|---|
| Round Nylon | .080 in | Grass, light weeds | Medium | Low |
| Square Nylon | .095 in | Thick weeds, light brush | High | Medium |
| Twisted (Helical) | .095 in | Heavy brush, small saplings | High | High |
| Serrated (Jagged) | .080 in | Mixed vegetation | Medium‑High | Medium‑High |
Tips for Optimal Performance
- Wind the line tightly and evenly; gaps between coils cause the line to bind.
- Use the exact length recommended by Stihl; too much line creates excess drag, too little reduces feed reliability.
- Store spare line in a cool, dry place to prevent moisture absorption, which makes nylon brittle.
- Inspect the bump knob spring periodically; a weakened spring leads to inconsistent feed.
- After each use, wipe the head clean to prevent grass buildup that can impede the bump mechanism.
Common Mistakes and How to Avoid Them
Even experienced users can make errors that lead to poor line feed or damage. The table below summarizes the most frequent mistakes and the corrective actions.
| Mistake | Result | Correction |
|---|---|---|
| Winding line opposite the arrow | Line feeds backward or jams | Always follow the arrow direction on the spool |
| Using line that is too thick | Head binds, motor strains | Stick to .080‑.095 in as specified |
| Leaving coils loose | Uneven feed, line breakage | Wind tightly, keep each turn snug |
| Overtightening the bump knob | Spring compresses fully, no bump action | Tighten until snug, then back off a quarter turn |
| Forgetting to disconnect spark plug | Accidental start, injury risk | Always disconnect before servicing |
Maintenance After Restringing
Once the new line is installed and feeding correctly, a few quick maintenance steps will keep the FS 40C running smoothly:
- Check the air filter and clean or replace it if dirty; a clean filter ensures proper engine performance.
- Inspect the spark plug for fouling; replace if the electrode is worn.
- Lubricate the drive shaft and gear case according to the service schedule in the manual.
- Examine the cutting head for cracks or excessive wear; replace the head assembly if the housing is damaged.
- Store the trimmer in a dry location, preferably hanging, to avoid moisture buildup in the head.
Troubleshooting Common Issues
If the line still does not feed after a proper restring, consider these troubleshooting steps:
- Line not advancing: Verify the bump knob rotates freely; a stuck knob prevents the spring from releasing line.
- Line breaks immediately: The line may be old or exposed to UV; replace with fresh line.
- Excessive vibration: The spool may be unbalanced; rewind the line ensuring even distribution.
- Engine stalls under load: The line diameter may be too large for the engine; switch to a smaller diameter.
Most issues resolve with a careful re‑inspection of the winding direction, line length, and head cleanliness.
Frequently Asked Questions
How often should I replace the trimmer line?
Replace the line whenever it becomes frayed, shortened to less than 2 inches, or when cutting performance noticeably declines. For typical residential use, this may be every 2‑4 hours of operation.
Can I use any brand of trimmer line?
Yes, as long as the diameter and shape match Stihl’s specifications for the FS 40C. Quality brands tend to last longer and feed more consistently.
Why does the line keep jamming in the head?
Jamming is usually caused by winding the line in the wrong direction, using a line that is too thick, or debris inside the head. Clean the head thoroughly and rewind following the arrow.
Do I need to lubricate the bump knob?
No lubrication is required; the knob relies on a dry spring mechanism. Adding oil can attract dirt and cause the knob to stick.
What is the correct line length for the FS 40C?
Stihl recommends approximately 10‑12 feet (3‑3.5 m) of .080‑inch line for the FS 40C. Always verify the exact figure in your model’s manual.
Can I use a dual‑line head on the FS 40C?
The FS 40C is designed for a single‑line fixed head. Installing a dual‑line head would require a different shaft adapter and is not recommended by Stihl.
How do I know if the bump knob spring is worn?
If the knob feels loose, does not return after tapping, or the line feeds erratically, the spring may be fatigued. Replace the spring or the entire bump knob assembly.
Is it safe to operate the trimmer without the bump knob?
No. The bump knob retains the spool and spring; operating without it can cause the spool to eject, creating a serious hazard.
Conclusion
Mastering how to string a Stihl weed eater Fs40c is a straightforward process when you understand the head design, use the correct line, and follow a disciplined step‑by‑step routine. By preparing the right tools, observing safety precautions, winding the line in the proper direction, and performing post‑restring maintenance, you ensure reliable line feed and extend the service life of both the cutting head and the engine. Keep this guide handy for future reference, and you’ll spend less time wrestling with tangled line and more time enjoying a clean, well‑trimmed lawn.