For the full breakdown, see our best Electric Trimmer for Bald Head guide.
Replacing a trimmer head is a 10-minute job taking no special skill, but the thread direction trips up most DIYers. Most string trimmer heads spin counterclockwise, meaning shaft threads are reversed — loosen the old head by turning it the same way it spins during use, not “lefty-loosey.” The procedure below applies to nearly every brand, with easy steps in order.
Safety First and Tools You Need
Before touching the head, isolate the power source completely. For a cordless model, remove the battery. For gas, pull the spark plug boot off. For corded electric, unplug it. This is non-negotiable — the head can spin even when the trigger isn’t pressed.
You’ll likely need a locking pin or Allen key (often included), gloves, and possibly a wrench. Most trimmers have a small hole in the shaft near the head where a pin slides in to lock it. Ensure your trimmer has the correct deflector installed before adding a new head — an undersized guard can expose you to flying debris.
Removing the Old Trimmer Head
Once isolated, insert the locking pin through the shaft hole to stop rotation. On some models, hold the shaft with one hand while turning with the other; on others, a pin or Allen key locks it. If the trimmer came with a tool, it fits the locking hole.
With the shaft locked, remove the old head by turning it in the direction it rotates while running. For STIHL, most heads like the AutoCut C 25-2 unscrew counterclockwise. On the AutoCut C 5-2, it’s slightly different: turn clockwise a couple of turns, push the tap head down, and continue clockwise until it releases. If stuck, a sharp tap on the wrench or penetrating oil soak usually breaks the grip — but never use heat near a fuel tank.
Keep gloves on during removal. Old line and head can have sharp edges, and a seized head can slip suddenly.
Installing the New Trimmer Head
Install the new head by threading it opposite the running direction — the reverse of removal. For most STIHL heads like the AutoCut C 25-2 and PolyCut C 20-3, that means clockwise. Husqvarna’s A15B and T35 heads follow the same logic: align the holes, lock the output shaft with the pin, and thread the new head on opposite the running direction.
Hand-tighten until it seats, then give a firm final turn while the shaft is locked. Don’t over-tighten to the point of stripping threads; “snug plus a little” is right. Remove the locking pin before testing.
Compatibility matters more than most realize. Some heads are model-specific and not universal — one manufacturer notes G2 accessories won’t fit G1 platforms, so check your exact model before buying. For EGO, the AH3800 commercial head fits the STX3800, while the AH1522 fits the ST1510T and ST1510T-FC.
Battery, Line Loading, and Common Mistakes
On battery trimmers, the most skipped step is removing the battery first. It takes two seconds and removes any chance of the head spinning near your hand. On gas trimmers, spark plug boot removal serves the same purpose.
Before reinstalling, check the new head’s line-loading path. Many heads feed line by pushing a button and pulling through; others require disassembly. Load line evenly — one side longer than the other causes vibration that wears shaft bearings prematurely.
The most common mistake after installation is forgetting the locking pin is still in place when testing. Remove it, then give the head a quick spin by hand to confirm it moves freely and line feeds. If anything feels gritty or loose, re-check thread direction before forcing it.
If replacing a worn head on an older trimmer and the shaft feels loose or the head wobbles, that points to a worn drive shaft or bearing — a new head won’t fix that, and running it risks damage. Also, replace a cracked or missing deflector before running anything new.
FAQs
Which direction do you turn a trimmer head to remove it?
Turn the head in the direction it rotates while running. For most models, this is counterclockwise for removal. The threads are reversed on the shaft, so standard loosening usually tightens it further. Lock the shaft with the pin first, then turn in the running direction.
Why won’t my trimmer head come off?
The shaft is almost certainly not locked. Insert the locking pin or Allen key into the shaft hole, then apply firm, steady force in the running direction. If it still won’t budge, apply penetrating oil and wait 15 minutes. Avoid a pipe wrench unless absolutely necessary, as it can crush the head housing.
Are trimmer heads universal fit?
No, trimmer heads are frequently model-specific. Always verify compatibility against your exact trimmer model number before purchasing. Some brands, like EGO, use different fitments across their lineup, and generation mismatches (such as G1 versus G2) exist within the same brand. When in doubt, check the manual.
References & Sources
- STIHL. “How to Replace Trimmer Head and Line.” Official US guidance on removal and installation direction.
- STIHL. “How to Replace the Grass Trimmer Head.” Brand-level steps for locking the shaft and threading direction.
- Husqvarna. “How to Replace a Trimmer Head.” Self-service steps for A15B and T35 heads.