Lawn Care
How to String a Grass Trimmer: Bump Feed & Fixed Line Guide
The Hidden Cost of Improper Trimmer Stringing
Learning how to string a grass trimmer correctly is not just about keeping your lawn edges sharp; it is a critical maintenance task that directly impacts your equipment's lifespan. A poorly wound spool or incorrect line diameter forces the trimmer motor to work harder, drawing excess amperage. In modern 2026 brushless platforms—like the EGO Power+ 56V or Milwaukee M18 Fuel—this excess draw can trigger thermal shutoffs, degrade battery cells, or permanently burn out the stator. According to the U.S. Consumer Product Safety Commission, improper equipment maintenance, including using mismatched or jammed trimmer lines, is a leading cause of tool failure and user injury from flying debris.
This guide bypasses generic advice and provides exact measurements, spool-winding mechanics, and material science insights to help you string both bump feed and fixed line heads with professional precision.
Identify Your Trimmer Head Type
Before cutting any line, you must identify your trimmer head mechanism. Using the wrong stringing technique for your specific head will result in immediate jamming. Below is a comparison matrix of the three primary head types found on residential and commercial equipment.
| Head Type | Mechanism | Line Capacity | Best Application | Common Failure Mode |
|---|---|---|---|---|
| Bump Feed | Spring-loaded spool releases line when tapped on the ground | 15–30 feet (wound on spool) | Standard residential edging and weed clearing | Line welding due to friction heat; spring fatigue |
| Fixed Line | Individual pre-cut strands locked into a central hub | 2 strands (13–15 inches each) | Heavy brush, fence lines, and commercial gas trimmers | Strands slipping out of the locking eyelets under high RPM |
| Speed-Feed | Line is threaded through the eyelets and wound by rotating the base | 20–35 feet (wound internally) | High-volume commercial landscaping (e.g., Echo Speed-Feed 400) | Cross-threading inside the internal chamber |
Selecting the Right Line: Diameter and Geometry
The most common mistake homeowners make is buying the thickest line available, assuming it will cut better. Penn State Extension turfgrass experts note that line diameter must be matched to the trimmer's power output. Thicker line creates more aerodynamic drag, which starves the motor of RPMs.
Diameter Matching Guide
- 0.065-inch: Light duty. Strictly for 18V–20V electric trimmers (e.g., Black+Decker, Ryobi ONE+). Using thicker line will burn out these motors.
- 0.080-inch: Standard duty. The universal size for 40V–56V battery platforms and small 25cc gas trimmers.
- 0.095-inch: Heavy duty. Required for 80V platforms, 30cc+ gas trimmers, and thick, woody weeds.
- 0.105 to 0.155-inch: Commercial grade. Only for high-displacement gas backpack trimmers (e.g., Stihl FS 560 C-E).
The Impact of Line Geometry
Beyond thickness, the cross-sectional shape of the nylon dictates cutting efficiency and noise levels:
- Round: The standard. Cheap and durable, but tears grass rather than slicing it, leading to brown, frayed tips.
- Square / Multi-Sided: Features sharp edges that slice cleanly through thick weeds. Best for overgrown areas.
- Twisted: Reduces aerodynamic drag and noise significantly. Ideal for noise-restricted suburban areas.
- Serrated (Gator-style): Features micro-teeth that act like a chainsaw chain. Excellent for woody stalks and saplings, but wears down faster on concrete edging.
Step-by-Step: How to String a Bump Feed Trimmer Head
Bump feed heads require precise winding to prevent the line from fusing together. Follow these exact steps:
- Remove the Spool: Depress the locking tabs on the side of the spool housing and pull the spool out. Note the direction of the existing line before removing it.
- Clean the Chamber: Use compressed air or a brush to clear grass clippings and dirt from the inner hub. Debris here causes the spool to drag, preventing line advancement.
- Anchor the Line: Cut 20 feet of new line. Locate the small anchor hole in the center divider of the spool. Feed the line through until you have equal lengths on both sides (for dual-line spools).
- Follow the Arrow: Look for the embossed arrow on the spool flange. Critical: Wind the line tightly in the direction of the arrow. If you wind it backward, the line will not feed out when you bump the head.
- Maintain Tension: Keep heavy tension on the line as you wind. Loose winding allows the bottom layers to shift and bind the top layers, causing a permanent jam.
- Lock in the Notches: Leave exactly 6 inches of slack on each side. Snap the line into the temporary locking notches on the spool's outer rim.
- Reassemble: Feed the 6-inch tails through the metal eyelets on the outer housing. Align the spool with the housing tracks, push down firmly until it clicks, and pull the lines to release them from the temporary notches.
Warning: The Friction Welding Effect
If you overlap the line while winding a bump feed spool, the friction generated at 7,000 RPM will melt the nylon strands together. This creates a solid plastic block inside the spool that cannot be unwound. Always wind side-by-side in the designated upper and lower channels.
Step-by-Step: How to String a Fixed Line Head
Fixed line heads (common on Stihl and Husqvarna commercial brushcutters) do not use a spool. They rely on pre-cut strands locked into a central hub.
- Measure and Cut: Cut two pieces of heavy-duty line (usually 0.105-inch or thicker). Measure exactly 15 inches for each piece. Longer strands will cause severe vibration and imbalance at high RPMs.
- Fold and Insert: Take one 15-inch strand and fold it in half, creating a 7.5-inch loop at the center.
- Lock into the Hub: Push the folded loop into the designated eyelet on the trimmer head until it clicks or bottoms out. The internal cam or metal clip will grab the loop, securing it against centrifugal force.
- Repeat and Balance: Insert the second strand into the opposite eyelet. Ensure both strands are of equal length to maintain the gyroscopic balance of the cutting head.
Pro Maintenance: The Hygroscopic Nylon Secret
Most users do not realize that trimmer line is made of extruded nylon, a hygroscopic polymer. This means it naturally absorbs moisture from the air. When nylon dries out over a winter in a hot garage, it becomes brittle and will shatter upon hitting a fence or concrete curb.
The Fix: Before stringing your trimmer for the spring, soak the coiled line in a bucket of warm tap water for two hours. The nylon will absorb the moisture, restoring its factory flexibility and impact resistance. For long-term storage, keep your spare spools in a sealed Ziploc bag with a damp paper towel to maintain optimal moisture content.
Troubleshooting Common Stringing Failures
Line Won't Advance When Bumped
This is rarely a broken spring. It is almost always caused by 'line crossover' inside the spool, where the top layer of line slips under the bottom layer, creating a physical lock. Remove the spool, unwind it completely, and rewind with strict, even tension.
Line Melts and Breaks at the Eyelet
If your line keeps snapping exactly where it exits the metal eyelet, the eyelet is likely worn down with a sharp, jagged groove cut into the metal by years of friction. Use a small round metal file to smooth the inside of the eyelet, or replace the trimmer head housing if the wear is severe.
Excessive Vibration After Stringing
Vibration means the head is out of balance. For fixed line heads, this means one strand is longer than the other. For bump feed heads, it means one side of the spool has more line wound onto it than the other, or the line has melted into a heavy lump on one side. Re-string with perfectly measured, symmetrical lengths.
