The Basic Process: What Happens When You Open the Head

Replacing trimmer string takes about five minutes once you know where the string sits and how the spool turns. Most trimmers use one of two designs: a fixed spool that you wind by hand, or an automatic or bump-feed head where you straightforward insert pre-wound string and close the cover. The string itself costs between $3 and $15 per spool depending on thickness and material — thicker string lasts longer but uses more power, so match what your trimmer originally came with.

Before you start, unplug the trimmer or remove the battery. Locate the trimmer head — the round part at the end of the shaft where the string comes out. Most heads have a cover held on by a single bolt, clip, or snap. Turn the bolt counterclockwise or press the clip inward to release it. The cover will come off, and you will see either a plastic spool wound with old string or a pre-wound cartridge sitting in a housing.

Key Takeaways

  • Most trimmer heads open with a single bolt or clip; remove it and the cover comes free, exposing the spool or cartridge underneath.
  • Fixed spools require you to wind new string by hand in the correct direction, usually counterclockwise, and anchor each end in a notch.
  • Bump-feed and automatic heads use pre-wound cartridges that you drop in and snap closed — no winding required.
  • String thickness matters: thicker string cuts better but drains battery faster, so match your trimmer's original specification.
  • After reassembly, test the string by running the trimmer briefly over concrete to wear it to the correct length.

Removing Old String from a Fixed Spool

If your trimmer has a fixed spool, you will see string wound around a plastic bobbin in the center of the head. Pull out any remaining old string — it usually comes free in one or two pieces. If string is stuck, use a flathead screwdriver to pry it loose from the notches where it anchors. Once the spool is clear, look for two small notches or slots on opposite sides of the spool rim. These are where the new string will anchor.

Some fixed spools have a center bolt holding them in place. If yours does, remove it with the correct wrench or socket — usually 8 mm or 10 mm — and lift the spool out. Others are fixed and do not come out; you wind the string directly onto them while they sit in the head. Check your trimmer's manual or look at the spool itself for an arrow showing which direction to wind. Most wind counterclockwise when you are looking down at the head from above.

Winding String on a Fixed Spool

Cut a length of new string — typically 20 to 30 feet, though your manual will specify. Lay the spool flat on a work surface if it came out, or leave it in the head if it is fixed. Find one of the two anchor notches and insert one end of the string into it, pushing it in until it catches. Now wind the string tightly around the spool in the direction the arrow indicates, keeping tension so the coils sit flat and do not overlap. Wind until you have about 6 inches of string left, then insert the other end into the second anchor notch.

The string should now be held at both ends and wound neatly around the spool with no loose coils. If you wound the spool out of the head, reinstall it by aligning any tabs or grooves and pushing it back into place, then replace and tighten the center bolt. If the spool stayed in the head, you are ready to close the cover.

Installing a Pre-Wound Cartridge in Bump-Feed or Automatic Heads

Bump-feed and automatic heads use cartridges — plastic spools of string that come pre-wound from the factory. When the cover is off, you will see a housing with a central post or spindle. The cartridge slides onto this post. Align the cartridge so any tabs or grooves match the housing, then push it straight down until it seats fully. You should hear or feel a click when it is seated correctly.

Some cartridges have a small tab or lever on top. If yours does, make sure it is in the correct position — usually pointing toward a mark on the housing — before you close the cover. Once the cartridge is seated, close the cover by aligning it with the head and pressing or snapping it into place. Tighten any bolt or clip that holds it. Do not force the cover; if it does not close smoothly, the cartridge may not be fully seated.

Reassembling the Trimmer Head and Testing

After the string is installed and the cover is closed, plug in the trimmer or reinstall the battery. Hold the trimmer at a safe distance from your body and run it briefly over concrete or a hard surface for two to three seconds. The string will wear down to the correct cutting length automatically. Most trimmers have a metal blade or guard that cuts the string to length as it spins.

If the string does not come out of the head or if it is uneven, stop and check that the cover is fully seated and the bolt or clip is tight. If the string is still stuck, open the head again and make sure it is not tangled or pinched under the cover. Once the string is cutting evenly and the head runs smoothly, you are done.

Choosing the Right String Thickness and Material

Trimmer string comes in different diameters, usually between 0.065 and 0.155 inches. Thicker string cuts tougher weeds and lasts longer, but it draws more power and will drain a battery faster on cordless models. Thinner string is lighter on the motor and battery but wears out quicker and may not cut thick stems. Check your trimmer's manual or the original packaging for the recommended thickness — using string that is too thick can strain the motor or cause the head to jam.

Most string is made of nylon, which is durable and affordable. Some brands offer twisted or serrated string that cuts slightly better, but the difference is small. Stick with the thickness your trimmer was designed for unless you have a specific reason to change — for example, if you are only trimming thin grass and want to extend battery life.

Frequently Asked Questions

How do I know which direction to wind the string?

Look for an arrow printed or molded into the spool itself — it shows the correct winding direction. If there is no arrow, check your trimmer's manual. Winding in the wrong direction will not damage anything, but the string may not feed correctly or may come loose.

Can I use a different thickness of string than the original?

You can use thicker string if your trimmer manual allows it, but thinner string is not recommended because it wears out faster and may not cut effectively. Thicker string than specified can strain the motor or jam the head. Always check the manual first.

What if the string keeps coming loose after I wind it?

Make sure both ends are fully inserted into the anchor notches — they should click or catch audibly. If the notches are worn or damaged, the string may not hold. You can also try winding the string slightly tighter, though do not force it so hard that you crack the spool.

How often do I need to replace the string?

It depends on how often you trim and what you are cutting. Light weekly use on grass might need string replacement every few weeks, while heavy use on thick weeds could require it weekly. You will know it is time when the trimmer stops cutting cleanly or the string becomes very short.

Is it cheaper to buy pre-wound cartridges or wind my own string?

Bulk string for hand-winding is usually cheaper per foot, but cartridges are more convenient and come ready to use. If you trim frequently, bulk string saves money over time. If you trim occasionally, a cartridge is simpler and the cost difference is small.