Last updated: October 2026 · Written by the LawnVibe editorial team.
A worn-out trimmer head turns every edging job into a fight: line that will not feed, a bump knob that sticks, or vibration that gets worse every time you use it. When new line and a new spool do not fix the problem, it is time to replace the trimmer head itself. This guide explains how to tell when the head is finished, how to remove it safely, how to match a replacement to your trimmer and how to install it so it runs smoothly.
If you only need fresh line, see our guide on how to wind trimmer line on a spool or how to replace the line on a trimmer.
When to replace the trimmer head
| Sign | What it means |
|---|---|
| Line will not advance even with a fresh spool | Worn bump knob, broken spring or damaged eyelets |
| Cracked or melted housing | Head damaged by impact or heat; replace |
| Eyelets worn into grooves | Line cuts itself off; replace eyelets or head |
| Heavy vibration after a hit | Head or arbor bent or cracked |
| Spool will not seat or lock | Broken tabs or worn housing |
If line will not advance but the head looks fine, try troubleshooting first; our guide on fixing a string trimmer that is not feeding line covers wound-too-tight spools, welded line and other simple causes.
Safety first
- Gas trimmers: turn off the engine, let it cool and disconnect the spark plug wire.
- Battery trimmers: remove the battery.
- Corded trimmers: unplug the cord.
- Wear gloves; worn heads can have sharp edges.
Types of trimmer heads
| Head type | How it works | Best for |
|---|---|---|
| Bump feed (OEM) | Tap the head on the ground to release line | Most users; matches the trimmer exactly |
| Automatic feed | Releases line automatically when it shortens | Light trimming, battery trimmers |
| Fixed line | Pre-cut lengths of line inserted into the head | Heavy line, tough weeds, no winding |
| Load-and-go / speed-feed | Feed line straight through and twist to wind | Users who hate winding spools |
| Universal replacement heads | Fit many brands with adapters | When OEM heads are unavailable or costly |
How to remove the old trimmer head
Heads attach in two common ways: screwed onto a threaded arbor, or held by a nut or bolt through the center.
Step 1: Remove the spool
Press the release tabs or unscrew the bump knob and take out the spool and spring. This exposes the center of the head and makes it easier to grip.
Step 2: Lock the shaft
The drive shaft must be held still while you unscrew the head. Many trimmers have a hole in the gearhead or a slot behind the head; insert a locking pin, a hex key or a small screwdriver to stop it turning. Some trimmers come with a locking tool.
Step 3: Unscrew the head
Many gas trimmer heads use a left-hand thread, so they come off by turning clockwise, which is the opposite of what you might expect. Battery trimmers vary. Check the manual or look for arrows molded on the head. Grip the head firmly, or use a wrench on the hex if it has one, and turn.
Step 4: Inspect the arbor and washers
Note the order of any washers, adapters or cups behind the head. Clean grass and debris from the arbor, and inspect the threads. Damaged threads should be repaired before installing a new head.
Choosing a replacement head
Original manufacturer heads
An OEM replacement head for your exact model is the easiest choice. It fits the arbor, uses the right spool and keeps the trimmer balanced. Search by your trimmer's model number in the manufacturer's parts catalog.
Universal heads
Universal heads come with several adapters to fit different arbors. To choose one:
- Find your arbor thread size and direction, such as a metric bolt or nut with a left- or right-hand thread.
- Check the head's compatibility list for your brand and model.
- Confirm the line diameter the head accepts matches your trimmer's power.
- Choose a weight and size suitable for the engine or motor.
Fixed-line and speed-feed heads
Fixed-line heads hold pre-cut pieces of heavy line and are popular for thick weeds and fence lines. Speed-feed or load-and-go heads let you thread line straight through and twist to wind it, which saves time on refills.
Considering an automatic-feed trimmer
If you are tired of bump heads altogether, many newer trimmers feed line automatically. Our guide to the best trimmer with automatic line feed compares models that take the hassle out of line changes.
How to install the new trimmer head
- Place any washers or adapters on the arbor in the same order they came off, or as the new head's instructions specify.
- Lock the shaft with the pin or tool.
- Thread the head on by hand in the tightening direction, which is usually counterclockwise for left-hand threads. Make sure it starts straight.
- Tighten firmly with your hand or the supplied wrench. Do not overtighten plastic heads.
- Remove the locking tool.
- Install the spool and line following the head's instructions.
- Spin the head by hand to check it runs true and does not rub the guard.
- Reconnect power and run the trimmer briefly at low speed, then full speed, checking for vibration.
Matching line size and the guard cutter
The new head should use a line diameter your trimmer can handle. Heavier line demands more power; too heavy a line on a small trimmer bogs the motor and wears it out. Also check that the line cutter blade on the guard trims line to the correct length for the new head. Line that is too long overloads the engine; too short reduces cutting width.
Replacing heads on battery trimmers
Battery trimmers often use heads specific to the brand and motor. Many use a right-hand thread or a nut-and-washer system rather than the left-hand threads common on gas trimmers. Some battery heads are designed around a particular line diameter and length to protect the motor and battery from overload, so a heavier aftermarket head can shorten run time and trigger overload protection. Stick to the manufacturer's replacement heads or universal heads that list your exact battery model as compatible.
Replacing heads on attachment-capable trimmers
Split-shaft or multi-tool systems let you swap the trimmer attachment for an edger, pruner or blower. The trimmer head still screws onto the attachment's arbor, but check that you are working on the trimmer attachment's gearbox, not the power head. Store spare heads with the attachment so they stay matched.
Bump head maintenance between replacements
A new head lasts much longer with a little care:
- Clean grass and dirt from the head after each use, especially around the eyelets and spring.
- Rotate or replace metal eyelets when grooves form; many heads have replaceable eyelets.
- Avoid hitting concrete, stone and fences with the head itself. Let the line do the cutting.
- Use fresh line, since brittle old line breaks off inside the head and causes jams. Storing line in a bag with a damp sponge keeps it supple.
- Check the spring and bump knob when you reload line. A weak spring stops the head from feeding properly.
Troubleshooting after installing a new head
The head wobbles or vibrates: it may not be seated straight on the arbor, a washer may be missing, or line lengths on each side are uneven. Remove the head, check the washer order and reinstall, then trim both lines to equal length.
The head loosens during use: it may be threaded the wrong way for your trimmer, or not tightened enough. Confirm the thread direction and tighten firmly.
Line still does not feed: the line may be the wrong diameter for the new head, wound too tightly or welded together from heat. Rewind with the recommended line size and wind evenly.
The engine bogs more than before: the new head may be heavier or the line longer than your trimmer is designed for. Use lighter line or a head rated for your engine size.
The head rubs the guard: the adapter stack may be wrong. Check the instructions for the correct adapter combination for your model.
Upgrading to a different head style
Replacing a worn head is a good opportunity to switch styles. Homeowners who trim mostly lawn edges often prefer a speed-feed head for easier reloading. Those who tackle thick weeds along fences and ditches often move to a fixed-line head with heavy line. Make sure the trimmer has enough power for the style you choose.
Tools you need
- Locking pin, hex key or small screwdriver to hold the shaft
- A wrench sized to the head's hex, if it has one
- Work gloves and eye protection
- A brush or rag for cleaning the arbor
- The owner's manual or parts diagram for your model
- The new head, with its adapters and washers
Keep the old head's parts
Before throwing away the old head, save any reusable parts such as an undamaged spring, metal eyelets or the bump knob. They can fix the new head quickly if one of its parts fails mid-season.
Write down your head specs
Once you have identified the arbor thread size, rotation and line diameter, note them on a label inside the shed or in your phone. Next time, you can order the right head or line in minutes.
How much does a new trimmer head cost?
Replacement heads range from inexpensive OEM bump heads to more expensive universal and fixed-line heads with metal components. A head is usually a small fraction of the cost of a new trimmer, so replacing a worn head is almost always worthwhile when the rest of the trimmer runs well.
Common mistakes
- Turning the head the wrong way and stripping threads.
- Forgetting washers or adapters behind the head.
- Choosing a universal head without checking thread size and direction.
- Fitting heavy line on a light battery trimmer.
- Overtightening a plastic head and cracking it.
- Removing the guard, which also removes the line cutter and protection.
The U.S. Consumer Product Safety Commission provides general safety guidance on using string trimmers, including eye protection and guards.
Ready to compare models?
Done with bump heads? Compare the best trimmers with automatic line feed.
See our Best Trimmer With Automatic Line Feed: Top Picks 2026 picks
The bottom line
To replace a trimmer head, disconnect power, remove the spool, lock the shaft and unscrew the head, remembering many gas trimmers use left-hand threads. Choose an OEM head for your model or a universal head that matches your arbor thread and rotation, install it with the washers in the right order, tighten firmly and test for smooth running. A fresh head restores reliable line feed, reduces vibration and makes trimming far less frustrating.
Frequently Asked Questions
When should I replace a trimmer head instead of the spool?
Replace the whole head when the housing is cracked, eyelets are worn through, the bump knob or spring is broken, or line will not feed even with a new spool.
Which way do you turn to remove a trimmer head?
Many gas trimmer heads use a left-hand thread and come off clockwise, but it varies by brand and model. Check the manual or markings on the head.
Are universal trimmer heads any good?
Quality universal heads work well when you match the arbor thread size and rotation with the right adapter. Confirm your trimmer model is on the compatibility list.
How do I know my trimmer head thread size?
Check the owner's manual or the manufacturer's parts diagram for your model. You can also measure the arbor bolt or nut, or take the old head to a dealer.
Can I put a gas trimmer head on a battery trimmer?
Only if the manufacturer approves it. Heavy heads can overload battery motors and void warranties. Use heads rated for your trimmer.