How To Properly Install & Replace String Trimmer Line

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String trimmers are a truly versatile outdoor power tool, capable of cutting the grass in those areas along the side of a home or fencing, or around landscaping, that the lawn mower just can’t reach. Need to do some quick weeding or take out some small foliage? A string trimmer can make short work of that as well.

But as easy as a string trimmer is to use, it’s easier to become frustrated trying to figure out how to replace the trimmer line correctly. This article will address the proper way to replace and install the string trimmer line as well as how a trimmer uses that line to keep your yard looking great.

How does string trimmer line work anyway?

For the trimmer line to get the job done, the trimmer head needs to be able to spin freely. To do this, the crankshaft of the string trimmer engine extends out the side of the crankcase and connects to a centrifugal clutch which engages with the drive shaft attached to the trimmer head. When the engine runs at idle speed, the clutch prevents the driveshaft from rotating. When the trigger is pulled, a cable opens the throttle on the carburetor which accelerates the engine. This causes the clutch flyweights to extend outward and engage the drive shaft which rotates the trimmer head.

As the trimmer head rotates, the trimmer line spreads out and whips around fast enough to cleanly cut through grass, weeds, or low foliage. The size and thickness of the trimmer line vary depending on the trimmer model, but the most common diameters for residential use range from 65/100th of an inch to 95/100th of an inch. The trimmer line may be wound around a spool, or the line may be cut into small individual strips that are loaded into the head. Most wound trimmer heads have a bumper, or bump knob, that can be tapped on the ground to feed additional line out of the head as the line is worn down, although newer models have an “auto-feed” feature. If too much line extends out of the trimmer head, a blade located on the guard will trim the line as the head rotates.

Eventually, the trimmer head is going to run out of line. You could simply replace the entire head with one already pre-loaded with line, but that’s not very economical. The better choice is to purchase a replacement trimmer line and, depending on the model, re-spool that line onto the spool located inside the trimmer head or insert the short trimmer line pieces into the eyelets in the head. Now let’s learn how to install the trimmer line.

Installing & Replacing trimmer line

While the exact method for properly replacing trimmer line can differ depending on which string trimmer model you’re using, the following will offer some general guidance on how to correctly install the line:

  • If you own a recent Ryobi string trimmer model (like the RY253SS), you won’t even have to detach the trimmer head to replace the line. First make sure the bump knob arrows are aligned with the lower trimmer head housing eyelets, then feed the new trimmer line through one eyelet and out the other until an equal amount of line is on both sides of the head. You can now rotate the bump knob clockwise (the Ryobi model has a crank handle available to make this easier) while holding the trimmer head to wind the line around the spool installed inside the head. Keep winding until the line is within the guard assembly.
  • For most other models (such as the Poulan Pro Model PP338PT trimmer), you will need to detach the trimmer head or housing from the drive shaft in order to replace the weed wacker line. Often, you can simply depress two retaining tabs on either side of the housing to release. Lift out the spool and remove any excess line. You can use a small brush to clean away any debris from the spool, eyelets, or the trimmer head housing. Following the manufacturer’s instructions, choose the appropriate size line for your trimmer and cut it to the recommended length – typically six to eight feet. Fold the line in half, making sure one end extends approximately five inches longer than the other end. Confirm the spool is properly orientated, then insert the loop into the notch in the spool and wrap the line clockwise around the spool, keeping the line taut and making sure it’s being wrapped evenly on both sides of the central spool partition. As you near the end of the line, place the short end into one of the notches in the top portion of the spool, allowing approximately six inches of line to be exposed. Continue to wrap the longer end around the spool and slide it into the notch directly opposite the one the short end is inserted into. Reposition the spool in the trimmer head so the notches align with the eyelets. While holding the spool in place, release both ends of the line from the spool notches and insert them into the eyelets. Reinstall the trimmer head. If either end of the line extends beyond the guard assembly, you should trim the line back before use.
  • If your model uses small individual strips of trimmer line like the Echo Model GT225 trimmer does, you should insert the line into the eyelets on the outside of the trimmer head so approximately one inch of line protrudes through the inside of the head when replacing the string trimmer line.

What if the trimmer line won’t feed?

Let’s say you’ve replaced the trimmer line, but the line is simply not feeding. Here are the top four reasons why your string trimmer may be letting you down:

1) Worn-out trimmer head  – The trimmer head may be worn out and unable to spin properly. Fortunately, trimmer heads are normally sold separately and are relatively easy to replace. Keep in mind, if the threads of the drive shaft are stripped, you’ll want to replace both the head and shaft at the same time. Even if the old head seems to be spinning fine, it may not be in good enough condition to properly feed the trimmer line. Before replacing the head, first try to clear out any grass or debris that may be clogged in the assembly; you should fully remove the head to clean it thoroughly.

2) Damaged trimmer head housing – On many models, the trimmer head is secured in a separate housing. If the housing is cracked or chipped, the line may be unable to feed.

3) Defective bump knob – Many trimmers will use a bump knob that can be tapped on the ground to activate a spool inside the trimmer head to issue more line. If the bump knob is defective (or the spool is damaged, for that matter), the line won’t feed properly. Again, both of these parts can be replaced to fix the problem.

4) Wrong size trimmer line – Not all trimmer line is equal as the size and thickness of the line vary depending on the trimmer model. As noted earlier, the most common diameters for residential use range from 65/100th of an inch to 95/100th of an inch, but you need to make sure you’re using the right line for your model. Otherwise, the line can become jammed inside the head.

Find the replacement string trimmer line you need and more at Repair Clinic

Now you just need to find the correct line for your string trimmer, right? takes out the guesswork. Now that you know how to install trimmer line, simply enter the full model number of your string trimmer in the Repair Clinic website search bar, then select “Trimmer Head” from the “Part Category” filter followed by “Trimmer Line” from the “Part Title” filter to identify the specific line suited for your trimmer. Looking to replace a defective bump knob or damaged trimmer head housing? Repair Clinic stocks original manufacturer replacement string trimmer parts from all the top brands, including Craftsman, Echo, Husqvarna, Kawasaki, MTD, Poulan Pro, Ryobi, Toro, and Troy-Bilt.

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