The basic process: remove the old head, install the new one, and wind the line
Replacing a weed eater head means unscrewing the spent head from the shaft, screwing on a new one, and loading it with trimmer line. Most heads unscrew by hand or with a wrench—the direction depends on whether your model has a reverse-threaded connection. Once the new head is tight, you wind the line into the spool, snap the cover back on, and you're ready to trim. The whole job takes 10 to 15 minutes and requires no special tools beyond what came with your trimmer.
The exact steps vary slightly between bump-feed heads (where you tap the ground to release more line) and automatic heads (where line feeds as it wears). Both types follow the same removal and installation process—the difference is in how you load the line.
Key Takeaways
- Most weed eater heads unscrew by turning counterclockwise by hand, though some require a wrench or an Allen wrench to hold the shaft steady.
- Check your trimmer's manual or the old head itself to confirm the thread direction, because a few models use reverse threads that turn clockwise.
- New heads come empty; you must wind trimmer line into the spool yourself, following the arrows molded into the spool to get the direction right.
- Bump-feed and automatic heads install the same way but load line differently—bump-feed requires you to wind it manually, while automatic heads have a spring-loaded mechanism.
- If the head spins freely when you try to unscrew it, use a wrench or Allen wrench to hold the shaft so the head can turn against it.
Gather the right tools and materials before you start
You need the replacement head itself, trimmer line (usually 0.080 or 0.095 inch diameter—check your manual), a wrench or Allen wrench that fits your trimmer's shaft, and your trimmer's manual or the old head as a reference. Most trimmers come with a wrench sized for the head; if yours didn't, a crescent wrench or adjustable wrench works. An Allen wrench is only necessary if your shaft has a hex socket to hold it steady while you unscrew the head.
Lay everything on a clean, flat surface so you don't lose small parts. If you're replacing the head on a gas trimmer, let it cool for at least 30 minutes before you start. Unplug electric trimmers or remove the battery.
Remove the old head by turning it counterclockwise
Most weed eater heads have standard threads and unscrew counterclockwise. Grip the head firmly with your hand or wrench and turn it away from you. If it's stuck, wrap a cloth around it for better grip, or soak the connection with penetrating oil and wait 10 minutes. Do not force it—if it won't budge, you may have a reverse-threaded head (see the next section).
If the head spins but won't unscrew, the shaft is turning with it. Use a wrench or Allen wrench to hold the shaft steady while you turn the head. On many trimmers, the shaft has a flat spot or hex socket near the head; fit your wrench there and hold it still while you turn the head with your other hand. Once the head is loose, unscrew it by hand the rest of the way and set it aside.
Check for reverse threads if the head won't turn counterclockwise
A small number of trimmer models—particularly some older or commercial-grade trimmers—use reverse threads on the head. This means the head unscrews clockwise instead of counterclockwise. Before you force anything, look at the old head or your manual to confirm the thread direction. If the manual shows a clockwise arrow or says "reverse thread," turn the head clockwise to remove it.
Reverse-threaded heads are rare on consumer trimmers, but they exist. If you're unsure and the head won't budge counterclockwise after a reasonable effort, stop and check the manual or contact the trimmer's manufacturer. Forcing the wrong direction can strip the threads on the shaft.
Screw the new head on and tighten it firmly
Thread the new head onto the shaft by hand, turning it in the same direction you unscrewed the old one (counterclockwise for standard threads, clockwise for reverse). Hand-tighten it until it's snug, then use your wrench to tighten it another quarter turn. Do not over-tighten—the goal is a firm connection that won't loosen during use, not maximum force.
If the head spins while you're trying to tighten it, use your wrench or Allen wrench to hold the shaft steady, just as you did during removal. Once it's tight, give it a gentle tug to confirm it won't come loose.
Wind trimmer line into the spool the correct direction
New heads come with an empty spool. Look for arrows molded into the spool itself—they show which direction to wind the line. Cut a length of trimmer line (usually 20 to 30 feet, depending on your spool size), find the small notch or slot at the edge of the spool, and insert one end of the line there. Wind the line tightly and evenly in the direction the arrows point, keeping tension on the line as you go so it doesn't bunch or overlap.
Wind until you reach the end of the spool or until the line fills the spool to the edge. Leave about 6 inches of line free, then insert the free end into the slot on the opposite side of the spool to hold it in place. Some spools have two slots—one for each end of the line. If yours does, wind the first half of the line into one slot, then wind the second half from the center outward into the other slot.
Once the line is wound, snap the spool cover back onto the head. The cover usually clicks into place or screws on—check your manual if you're unsure. Pull gently on the line to confirm it feeds out smoothly when you tug it.
Load line into automatic heads differently than bump-feed heads
Automatic heads have a spring-loaded mechanism that feeds line as it wears, so you don't wind the line into the spool the same way. Instead, you insert the line into a feed hole or slot, and the spring pulls it into the spool automatically. Check your manual for the exact location of the feed hole and the length of line to insert—it's usually 12 to 18 inches. Insert the line, snap the cover on, and the head is ready to use.
Bump-feed heads, by contrast, require you to wind the line manually as described in the previous section. When the line wears down during use, you tap the head on the ground to release a small amount of pre-wound line. If your new head is automatic and you're used to bump-feed, or vice versa, the loading process will feel different—that's normal and expected.
Frequently Asked Questions
How do I know what size trimmer line to buy?
Check your trimmer's manual or the old head itself—the line diameter is usually printed on the spool or packaging. Common sizes are 0.080 inch and 0.095 inch. Using the wrong size can cause the head to jam or feed line poorly. When in doubt, take the old head to a hardware store and match it to the replacement line.
What if the new head won't screw on all the way?
Stop and check that you're turning it in the correct direction. If it's the right direction but still won't seat, the threads may be damaged or misaligned. Remove the head, inspect the shaft threads for dirt or damage, clean them with a dry cloth, and try again. If it still won't seat, the shaft or head may be defective and need replacement.
Can I reuse the old trimmer line in a new head?
Yes, if the line is still intact and the correct diameter. Unwind it from the old spool, straighten it, and wind it into the new spool following the direction arrows. If the line is cracked, brittle, or the wrong size, discard it and use new line.
Why does my new head keep loosening during use?
The head may not be tight enough, or the shaft threads may be worn. Remove the head and inspect the threads on both the shaft and the head for damage or debris. Clean them, reinstall the head, and tighten it firmly with your wrench. If it loosens again after a few uses, the threads are likely worn and the shaft or head may need replacement.
Do I need to replace the head if I just run out of line?
No. If the spool and head are still intact, you can reload them with new trimmer line. Only replace the head if the spool is cracked, the mechanism is broken, or the line won't feed properly even after reloading.