Quick Answer: How to String a Bump-Feed Weed Wacker
Remove the bump knob, lift out the spool, and clear old line. Cut fresh trimmer line to the length stamped on your spool, wind it onto the spool in the direction the molded arrow indicates, thread the free ends through the eyelets, and reinstall the bump knob. The trimmer is ready to use immediately after power is restored.
What Is a Bump-Feed Trimmer Head and How Does It Work?
A bump-feed trimmer head advances line on demand: tap the bump knob on the ground while the trimmer is running, and centrifugal force pulls a measured length of line out through the eyelets (the small exit holes on the head housing). Release the tap, and friction locks the line in place again.
This mechanism is why winding direction and line seating are critical. If the line is wound backward or the spool is seated upside-down, the centrifugal force works against the feed instead of with it — the knob taps but no line advances.
How Do I Know If My Head Is Bump-Feed?
- Bump-feed: Has a rounded plastic knob on the bottom. You tap it on the ground to extend line.
- Fixed-line / manual feed: No knob; line is cut to a fixed length and replaced as a whole.
- Automatic / auto-feed: Has a knob but advances line without tapping — line extends automatically when it wears short.
Confirm you have a bump-feed head before continuing. The steps below do not apply to automatic-feed or fixed-line heads.
Before You Start: Line Diameter, Length, and Head Type
Two decisions made before winding determine whether the job works: choosing the right line and knowing whether your head is single-line or dual-line.
Line Diameter
Trimmer line diameter is measured in inches or millimeters. Common sizes are 0.065 in, 0.080 in, 0.095 in, and 0.105 in. Using the wrong diameter causes two problems: line that is too thin breaks constantly; line that is too thick will not fit through the eyelets or may strain the motor.
Find your correct diameter by checking, in order:
- The label molded or printed on the trimmer head itself
- Your model’s owner’s manual (usually listed under “Specifications” or “Replacement Parts”)
- The manufacturer’s website product page for your specific model number
Do not assume a universal size. Diameter specs vary by model even within the same brand.
Line Length
The maximum line length the spool holds is typically stamped directly on the spool face. Cutting line longer than this limit causes overfilling, which jams the spool inside the housing and prevents the bump-feed mechanism from moving. Cut to the stamped limit — not longer.
Single-Line vs. Dual-Line Heads
| Feature | Single-Line Head | Dual-Line Head |
|---|---|---|
| Anchor slots on spool | 1 | 2 (opposite sides) |
| Eyelets on housing | 1 | 2 |
| Line pieces needed | 1 length | 2 equal lengths or 1 length folded at midpoint |
| Cutting speed | Slower | Faster; line wears more evenly |
Dual-line heads are more common on mid-range and heavy-duty string trimmers. Check how many eyelets your head housing has — that is the fastest way to identify your type.
Pre-Wound Replacement Spools
Some bump-feed heads accept a pre-wound replacement spool — a drop-in cartridge that comes already loaded with line. If your trimmer uses these, Steps 4 and 5 in the guide below are skipped entirely: remove the old spool, drop in the new one, thread the eyelets, and reinstall the bump knob. Check your owner’s manual or search your model number plus “replacement spool” to confirm availability.
How to String a Bump-Feed Weed Wacker: Step-by-Step
Step 1 — Disconnect Power
Before touching the head, eliminate all risk of accidental startup.
- Battery-powered trimmers: Remove the battery pack.
- Corded electric trimmers: Unplug from the outlet.
- Gas trimmers: Pull the spark plug boot off the spark plug. Do not skip this — a gas engine can fire if the blade or line catches on debris during handling.
Step 2 — Remove the Bump Knob and Lift Out the Spool
Look for an arrow or text on the underside of the bump knob indicating the removal direction — it is almost always counterclockwise when viewed from below. Turn the knob in that direction until it releases, then lift it off. The spool sits inside the housing beneath it; lift the spool straight out.
Tip: Some bump knobs are press-and-turn; others unscrew freely. If it resists, press the knob inward while rotating — that releases the locking tab on many heads.
Step 3 — Clear Old Line and Clean the Housing
Pull all remaining trimmer line off the spool and discard it. Inspect the spool and head housing for packed grass, dirt, and broken line fragments. A buildup of debris in the housing prevents the spool from seating fully, which causes the bump-feed mechanism to stick. Wipe clean before winding new line.
Step 4 — Cut the Line to Length
- Single-line head: Cut one piece of line at the length marked on the spool.
- Dual-line head (two separate pieces): Cut two pieces of equal length, each at the spool’s maximum.
- Dual-line head (one piece folded): Cut one piece at twice the spool’s maximum. Fold it at the midpoint to create a loop; that loop anchors at the center divider of the spool.
Tip: Measure against a tape measure, not by eye. Unequal lengths on a dual-line head cause one side to run out before the other, creating uneven cutting and frequent stopping.
Step 5 — Anchor the Line and Wind the Spool
This is the most failure-prone step. Execute in this order:
- Insert the line end into the anchor slot on the spool. Push it in until it seats — it should hold the line firmly when you tug lightly. On dual-line heads, insert one end into each of the two anchor slots on opposite sides of the spool.
- Locate the arrow direction indicator molded on the spool face. Wind the line in exactly that direction. Winding backward is the single most common stringing mistake — it causes the bump-feed to lock instead of advance.
- Keep each wrap tight against the previous wrap. Start at the bottom of the spool’s winding channel and work upward in a single layer before starting a second layer. Do not crisscross or stack wraps haphazardly — crossed wraps bind inside the housing and prevent line from feeding.
- Stop winding when 5–6 inches of line remain free. Do not wind all the way to the end.
- On dual-line heads with a center divider: wind each line piece in its designated half of the spool, keeping the two lines fully separated by the divider.
Tip: Many spools have small notches on the rim to temporarily hold the free end while you work. Clip the line into a notch so it does not unwind as you reassemble the head.
Step 6 — Seat the Spool and Thread the Eyelets
Lower the wound spool back into the head housing. Align any tabs or keyways on the spool with the matching slots in the housing — misalignment prevents the bump-knob from locking and causes the spool to rattle or fail to advance line.
Thread each free end of line through its corresponding eyelet on the housing. Pull each end through until the line seats fully and the spool drops flat in the housing with no gap. If you clipped the line into a holding notch in Step 5, release it from the notch before threading the eyelet.
Step 7 — Reinstall the Bump Knob
Press the bump knob down onto the housing and rotate it in the locking direction (opposite to the removal direction — usually clockwise when viewed from below) until it clicks or stops turning. Give it a firm tug to confirm it is locked. A bump knob that is not fully locked will eject from the head at operating speed.
Step 8 — Restore Power and Test
Reconnect the battery, plug in the cord, or reattach the spark plug boot. Start the trimmer and let it run for a few seconds, then tap the bump knob on a hard surface once. The line should advance by roughly 1–2 inches per tap. If it does, the stringing is correct.
Troubleshooting: Why Your Bump-Feed Head Is Not Advancing Line
Problem 1 — Line Welds or Fuses to Itself on the Spool
Cause: Trimmer line generates friction heat during use. Low-quality line or line wound too tightly traps that heat and causes adjacent wraps to melt and bond together — a condition called line welding or line fusing.
Fix: Replace fused line entirely; it cannot be separated safely. Use quality line rated for your trimmer’s specified diameter. Wind wraps firmly but not compressed. Store the trimmer in shade or indoors — sustained heat from sunlight accelerates fusing even when the trimmer is not running.
Problem 2 — Line Feeds Unevenly or One Side Runs Out Faster
Cause: On dual-line heads, unequal winding lengths or crossed wraps on one side cause one line to wear faster than the other. The shorter side runs out mid-session, leaving one line working and one absent — which also throws the head off balance.
Fix: Rewind. Measure both pieces to the exact same length before winding. Keep wraps parallel and confined to each spool half. Do not allow line from one half to cross the center divider.
Problem 3 — Bump Knob Taps But No Line Advances
Cause (most common): The spool is seated upside-down. The arrow on the spool face must point outward (visible when looking at the bottom of the assembled head). If the spool is flipped, the centrifugal mechanism works in reverse and line locks instead of advancing.
Second cause: The spring inside the bump-feed head is broken or missing. Without spring tension, the bump knob cannot compress and release properly.
Fix: Remove the bump knob and reseat the spool with the arrow facing outward. If that does not resolve it, inspect the spring: a broken spring requires replacing the spring alone (if sold separately for your model) or replacing the entire head assembly.
How Often Should You Restring a Bump-Feed Trimmer?
Restringing frequency depends on use intensity, line diameter, and the vegetation type being cut. As general guidance — not a manufacturer specification — residential users trimming lawns and light weeds typically need to restring every 3–5 trimming sessions. Heavy or commercial use cutting thick brush may require restringing every session.
The most reliable indicator is functional: restring whenever the bump knob no longer advances line after repeated tapping. That behavior signals the spool is empty or the remaining line is too short to exit the eyelets properly.
Thicker-diameter line generally lasts longer per session because it resists breakage against hard surfaces (fences, edging, rocks), but it also fills the spool faster, meaning fewer total feet per wind. The tradeoff is durability per cut vs. total sessions per spool load.
Inspect the spool every time you refuel (gas models) or swap batteries (cordless models). That habit catches low-line conditions before they strand you mid-job.
Frequently Asked Questions
Which direction do I wind trimmer line onto a bump-feed spool?
Wind in the direction the arrow molded on the spool face indicates — always. That arrow accounts for the rotation direction of your specific trimmer head. Winding against the arrow causes the bump-feed mechanism to lock line in rather than advance it. When in doubt, check the owner’s manual for your model, as rotation direction varies between clockwise- and counterclockwise-spinning heads.
How much free line do I leave out before reassembling the head?
Leave 5–6 inches of free line extending past the spool rim before threading the eyelets. Less than that can make the line difficult to grab and thread through the eyelet. More than 6–8 inches risks tangling inside the housing during reassembly. After the head is fully reassembled and the trimmer runs, the spinning action trims excess line to the correct cutting length automatically (or you can trim it manually with scissors).
Can I use any brand of trimmer line in my bump-feed head?
Brand does not matter — diameter and shape do. The line must match the diameter specified for your trimmer (found on the head label or owner’s manual). Shape (round, twisted, multi-sided) affects cutting aggression but is generally interchangeable as long as diameter is correct and the line fits through your eyelets. Avoid off-brand line with no stated diameter specification, as inconsistent thickness causes jamming and uneven wear.
What happens if I overfill the spool with too much line?
Overfilling jams the spool inside the housing. The excess wraps press against the housing walls and prevent the spool from moving when you tap the bump knob — so the feed mechanism cannot function. If the bump knob feels rigid and refuses to depress after restringing, overfilling is the most likely cause. Remove the spool and rewind, cutting the line back to the length stamped on the spool.
