What Is A String Trimmer Bump-Feed Head (Line Jam Fixes)
A string trimmer bump-feed head holds cutting line on a spool and releases a short length when you tap its bump button against a suitable surface while the head spins at the speed specified by the manufacturer. If it will not feed, stop and disconnect power before checking for tangled or oversized line, a misseated spool, debris, or damaged parts.
A bump head can seem stubborn: you tap it, nothing happens, and the weeds keep winning. The fix is usually not a harder hit. It is safer to understand how the head feeds line, then check the spool and its parts in order. I use that method to avoid turning a small jam into a cracked cap or a loose head.
Equipment Setup and Safety Preparation
A bump-feed head is the assembly at the end of the trimmer shaft that holds and feeds cutting line. Before using or opening one, confirm it fits the trimmer, identify its correct line and attachment parts, and isolate the power source. This prevents accidental startup and helps avoid damage from mismatched parts.
How a bump-feed head works
Inside the head, a spool stores one or more strands of trimmer line. When the rotating head’s bump button is tapped as directed, the mechanism lets the spool move enough to release line. A cutter on the guard trims the extra line to working length. The exact mechanism and operating speed vary by model.
Do not assume tapping the button while the head is stopped will feed line. Check the trimmer and head manuals for the specified operating method. Do not strike the head hard against concrete or another hard surface to force it. That can damage the bump button or housing.
Check fit, line, and safety first
A replacement head must match the trimmer’s shaft and gearbox connection. That includes the thread size and direction, or the specified adapter, as well as the shaft type and the head’s rated operating speed. There is no universal bump-head fit. Use the retainer, adapter, and washers specified for your model; never force a mismatched head onto the gearbox or substitute a bolt.
Use only a line diameter approved for both the head and the trimmer. Many consumer models use 0.080-inch (2.0 mm) or 0.095-inch (2.4 mm) line, but these are examples, not universal recommendations. Line that is too large may bind at the eyelets or on the spool.
Before work or inspection:
- Wear eye and face protection, hearing protection, gloves, long trousers, and sturdy footwear.
- Move people, pets, and breakable items beyond the area where debris could be thrown.
- Stop the engine and wait for the head to stop fully.
- On a gas trimmer, disconnect the spark-plug lead. On a cordless trimmer, remove the battery.
- Keep the guard installed during operation.
Field Trimming Operation and Bump-Feed Technique
Use a bump head only with the line, head, and trimmer combination approved for your equipment. During trimming, hold the tool as its manual directs and bump the button against a suitable surface only at the specified operating speed. If line does not feed, stop and isolate the power before inspecting the head.
Feed line without forcing it
A short tap may release line when the head is rotating at the manufacturer’s specified operating speed. The button is not a hammer target. Avoid repeated hard strikes, and do not try to pull line out while the head is running or the battery is connected.
If the head stops feeding, first note whether one strand or both strands are affected. That clue can point toward crossed line, a blocked eyelet, or a problem with the spool. It does not prove which part is faulty, so inspect the head before trying again.
| What you notice | Possible cause to check | Safer next step |
|---|---|---|
| No line comes out | Crossed or jammed line, full spool, blocked eyelet, or spool not seated | Stop, isolate power, open and inspect |
| One strand feeds but the other does not | One strand may be crossed, stuck, or blocked | Check both channels and eyelets |
| Line feeds, then stops | Winding may be uneven, or line may bind as the spool turns | Remove and reload line as directed |
| Head vibrates or wobbles | Damaged, loose, or incorrectly seated parts may be involved | Stop immediately and inspect before reuse |
Jam Removal, Head Inspection, and Line Reloading
A line jam is a blockage that prevents the spool from releasing line through the head’s eyelets. Common causes include crossed or uneven winding, too much line, debris, swollen or melted line, and damaged parts. Remove the jam only after stopping the trimmer and disconnecting its power source.
Open and inspect the head
Follow the head manual to remove the cap; head designs differ, so do not assume it twists off or releases in a particular direction. Keep track of the spool, spring, and any other parts as you open the assembly. Do not pry against the gearbox or modify the head to make it open.
Look for:
- Line tangled around itself or crossing between spool channels.
- Line that is melted, dirty, swollen, or stuck at an eyelet.
- Grass or grit packed around the spool.
- A spring that has moved out of place or appears broken.
- Cracks in the cap or spool, and damaged eyelets.
Clear loose debris. Replace damaged components with parts specified for the head; do not reuse a cracked cap or damaged spool. If you cannot identify how the spring or spool should sit, stop and check the manual rather than guessing.
Reload and reassemble correctly
Use the line diameter stated on the head or in its manual. Wind in the marked direction, keep each strand in its own channel, and leave the specified tail length. Do not cross the strands, overfill the spool, or pull the line so tight that it cannot unwind as designed.
Seat the spring and spool in their correct positions, route the line through the eyelets, and lock the cap fully. With power still isolated, check that the head turns freely by hand and that the retainer is secure. If anything binds, comes loose, or does not fit as shown in the manual, reopen and correct it before testing.
Common repair scenarios: When a head has line left on the spool but will not feed, an owner may find crossed winding or grit near the spool. When one side feeds and the other does not, the blocked strand or its eyelet is worth checking. These are diagnostic examples, not proof of a specific fault; inspection decides what needs repair.
Test after the jam is cleared
Reconnect the spark-plug lead or battery only after the head is closed and secure. Test in a clear area, with the guard installed and the head below waist level. Follow the manual for starting and bump-feed technique. If the line still will not feed, shut down and isolate power before reopening the head.
Stop at once if the head wobbles, vibrates unusually, cracks, or has a loose retainer. A mismatched retainer, damaged cap, poorly seated spool, or oversized line can create a risk of head failure or thrown debris. Replace defective parts before using the trimmer again.
Shutdown, Cleaning, and Seasonal Storage
After trimming, shut down the trimmer and wait for the head to stop and cool. Disconnect power before cleaning or inspecting it. Removing grass and grit can help prevent buildup, while a quick check of the line and head may reveal damage before the next use.
Wipe away debris without putting your hands near a moving head. Check that the cap, retainer, and guard remain secure and that the line is not badly worn or stuck. For storage, follow the trimmer and line manufacturer’s instructions; requirements can differ by model and line type.
Do not change fuel-oil ratios, idle speed, maximum speed, battery settings, or head-speed limits based on general advice. Those values are model-specific. Use the trimmer manual and head label, and do not apply chainsaw chain, bar, or cutting-tooth specifications to a string trimmer.
Fast troubleshooting checklist
When line jams again, use this order:
- Stop the trimmer and isolate the power source.
- Confirm the line diameter is approved for the head.
- Open the head as instructed and clear debris.
- Check for crossed, uneven, melted, swollen, or overfilled line.
- Inspect the spring, spool, eyelets, cap, and retainer for damage or poor seating.
- Reload in the marked direction and reassemble fully.
- Test in a clear area; stop if feed still fails or the head vibrates.
Frequently Asked Questions
What does a bump-feed head do?
It stores trimmer line on a spool and releases line when its bump button is tapped as directed while the head rotates at the specified operating speed.
Why will my bump head not feed line?
Possible causes include crossed or overfilled line, a blocked eyelet, debris, an incorrectly seated spool, or a damaged part. Stop and disconnect power before inspection.
Can I tap the head while the trimmer is off?
Do not count on it. Many bump-feed heads release line during rotation, and the correct method depends on the head. Check its manual.
Can I use 0.095-inch line in any bump head?
No. Use only the diameter approved for your specific head and trimmer. A line that is too large can bind in the spool or eyelets.
How do I know which way to wind the line?
Follow the arrows or winding marks on the spool or the directions in the head manual. Keep strands in their separate channels.
Should I hit the bump head harder if it will not feed?
No. Hard strikes can damage the head. Stop, isolate power, and check the line, spool, and eyelets instead.
Is it safe to remove the cap with the battery connected?
No. Remove the battery first. For a gas trimmer, stop the engine and disconnect the spark-plug lead before opening the head.
What if the head wobbles after I reload it?
Stop immediately and disconnect power. Check whether the spool and cap are seated and the retainer is secure. Replace damaged parts before reuse.
Can I fit any replacement bump head to my trimmer?
No. The attachment, thread direction or adapter, shaft type, and rated speed must match. Check the trimmer and head manuals.
When should I stop troubleshooting?
Stop if parts are cracked, the spring or assembly is unclear, the retainer will not secure, or the head still vibrates or fails to feed after correct reassembly. Do not run a damaged or loose head.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
