How to Replace String on a Stihl FS 45 Trimmer

The Stihl FS 45 is a popular choice among homeowners and landscaping professionals who need a reliable curved-shaft weed trimmer for maintaining their yards and properties. One of the most common maintenance tasks you’ll encounter is replacing the nylon cutting string, which naturally wears down over time due to friction against grass, rocks, and other obstacles. Understanding how to properly reload your trimmer head with fresh cutting line can save you time and frustration while keeping your equipment in optimal working condition.

Understanding Your Trimmer’s Cutting Line System

The Stihl FS 45 utilizes the AutoCut C 6-2 trimmer head, which represents a significant advancement in ease-of-use compared to older trimmer models. This innovative design eliminates the frustration of manually wrapping nylon line around a fixed spool while trying to maintain tension and alignment. Instead, the AutoCut system features a two-ring configuration with an intuitive loading mechanism that makes the restringing process straightforward for users of all skill levels.

The cutting line itself is made from durable nylon material that gradually wears away as you trim vegetation. The rate of line consumption depends on several factors, including the types of vegetation you’re cutting, proximity to hard objects, ground conditions, and your trimming technique. Rocky or uneven terrain accelerates line wear considerably compared to smooth lawn conditions.

Selecting the Correct Line Specifications

Before attempting to reload your trimmer, it’s essential to understand the proper line specifications for your particular model. The Stihl FS 45 accommodates nylon trimmer line in two primary diameter sizes: .080 millimeters and .095 millimeters. These measurements refer to the thickness of the cutting line.

The .080 millimeter size works well for general lawn maintenance and routine trimming tasks where you’re primarily cutting grass and light weeds. This diameter provides adequate cutting power for most residential applications while offering reasonable fuel efficiency. However, if you frequently encounter tougher vegetation, encounter rocky terrain, or trim near hard edges and objects, the .095 millimeter line offers increased durability and cutting strength. The thicker line withstands impact better and maintains its shape longer when striking hard objects, though it may consume slightly more fuel due to increased cutting resistance.

Preparing Your Trimmer for Line Replacement

Proper preparation is crucial before you begin the line replacement process. First, ensure that the engine has completely cooled if you’ve been using the trimmer recently. Operating or warm engines pose burn hazards, and a cool engine makes handling the equipment safer and more comfortable.

Next, position your trimmer in a way that provides comfortable and clear access to the trimmer head. Place the FS 45 on its engine block with the curved-shaft end pointing upward. This orientation gives you the best access to the AutoCut head and allows gravity to assist you during the line installation process. Having the trimmer head at a comfortable working height reduces strain on your back and arms and minimizes the likelihood of accidentally dropping or misaligning components.

Gather all necessary materials before beginning. You’ll need the replacement nylon line of the correct diameter, a pair of sharp scissors for cutting the line to length, and potentially a flat-blade screwdriver if you encounter difficulty removing stubborn components. Having everything organized and within reach prevents interruptions during the process.

The Ring Alignment Process

The AutoCut C 6-2 head features two distinct rings: an outer black ring and an inner white ring. These rings work together to hold and dispense the cutting line. To properly access the line storage area, you must align directional markers on both rings.

Locate the four arrow markings that appear on both the outer and inner rings. These arrows serve as alignment guides and must point in the same direction. Gently push inward on the white inner ring while holding the black outer ring steady. Continue pushing until the arrows on both rings align perfectly with each other. This alignment is essential because it allows the line to be properly accessed and installed in the correct storage slots.

Removing the Worn Cutting Line

Once the arrows are aligned, you should be able to pull out the old, worn nylon line from the trimmer head. In most cases, the line comes free with gentle pulling. However, if the old line resists removal or remains stuck, you may need to partially disassemble the trimmer head.

To disassemble the head, hold the black outer ring firmly and locate the center nut of the trimmer head. Using a large flat-blade screwdriver for leverage, turn the center nut counterclockwise to loosen it. This may require considerable force, especially if the nut hasn’t been removed recently, but patience and steady pressure will eventually free it. Once the center section is removed, you’ll have full access to extract any remaining old line.

After removing the spent line, reassemble the trimmer head by reversing these steps. Screw the center nut clockwise until snug, and realign the arrows on both rings. Test that the white inner ring moves freely through its full range of motion, as this movement is necessary for the line advancement mechanism to function properly.

Loading New Cutting Line Into Your Trimmer

Now comes the actual line installation. Begin by measuring and cutting a section of new nylon line approximately 13 feet in length. This specific length provides sufficient material to fill both line storage chambers within the AutoCut head while leaving a small margin for adjustment.

Take the 13-foot section of cut line and carefully thread it through the AutoCut head mechanism. The goal is to achieve equal lengths on both sides of the trimmer head. With the arrows still aligned, push the line through the central passage point until roughly half of the total length (approximately 6.5 feet) extends on each side. This balanced distribution ensures that both cutting edges work evenly and wear at similar rates during operation.

As you feed the line through the head, you should encounter some resistance. This resistance indicates that the line is properly seating in the storage chambers. If the line passes through with no resistance whatsoever, it may not be properly installed, and you should recheck your work.

Winding and Securing the Line

With the line properly threaded through the head, the final step involves winding the excess line around the internal spools. Hold the black outer ring of the trimmer head stationary while rotating the white inner ring counterclockwise. This winding action stores the excess nylon line in the designated chambers.

Continue winding the white ring counterclockwise until most of the excess line is wound up. You should stop when approximately 5 inches of line remains visible at the edge of the trimmer guard. This reserve length ensures you have adequate cutting surface for operational use while preventing excessive line from being exposed during trimming.

Once you’ve wound the line to the appropriate level, use your scissors to trim any excess line extending beyond the guard’s edge. Make clean, straight cuts perpendicular to the line to prevent fraying. The trimmed line should be relatively even on both sides of the trimmer head, with equal lengths ready to make contact with your yard or landscaping work.

Line Storage and Maintenance

Proper storage of trimmer line between uses extends its lifespan and maintains its optimal cutting characteristics. Nylon line degrades when exposed to ultraviolet radiation and excessively dry conditions. To preserve your trimmer line quality, store spools or coils of replacement line in a cool location away from direct sunlight.

An effective storage method involves keeping trimmer line in a sealed container with a small amount of water. The moisture keeps the nylon supple and flexible, preventing it from becoming brittle or losing its shape. Some users find it convenient to store their replacement line in sealed plastic bags with a damp cloth, though a dedicated storage container works equally well.

Regular inspections of your current line during operation help you catch wear issues before you run out of line entirely. If you notice the cutting line becoming noticeably shorter or the trimmer performing less effectively, it’s time to reload. Staying ahead of this maintenance prevents frustration when you’re ready to do yard work.

Troubleshooting Common Installation Issues

If you experience problems during the line replacement process, understanding potential issues helps you resolve them efficiently. If the line won’t feed through the head despite proper alignment, you may have debris or dried nylon fragments blocking the passage. Carefully inspect the internal channels and use a thin wire or compressed air to clear any obstructions.

If the white inner ring feels stiff or won’t rotate freely after installation, the line may be wound too tightly or improperly seated. Carefully unwind the line, recheck alignment, and attempt the installation process again with slightly less tension.

In cases where the trimmer head refuses to come apart for cleaning or old line removal, applying penetrating oil around the center nut and allowing it to soak for several hours can help. Never use excessive force that might crack plastic components or strip the threading.

Performance Tips After Line Replacement

After successfully replacing your trimmer line, a few operational guidelines help maximize your equipment’s performance. Run the trimmer briefly at idle to ensure the line is properly seated and the head rotates smoothly without wobbling or vibration. Minor adjustments to line balance may be necessary if one side appears longer than the other during this test run.

When beginning your first trimming session with new line, use a gentle touch. Avoid immediately tackling the toughest vegetation or testing the new line against rocks or hard objects. A gradual break-in period allows the line to settle properly in the storage chambers and ensures optimal performance once you increase to normal operating conditions.

When Professional Service Becomes Necessary

While line replacement is straightforward for most users, certain situations warrant professional service. If you’ve attempted multiple replacements and consistently experience installation difficulties, or if you notice unusual vibration or damage to the trimmer head itself, a qualified Stihl service technician can diagnose underlying issues.

The AutoCut head is generally durable, but wear to internal components or damage to the alignment system occasionally occurs. Professional service ensures these more serious issues are properly addressed rather than attempting workarounds that compromise safety or performance.

Frequently Asked Questions

Q: How often should I replace the trimmer line on my Stihl FS 45?

A: The frequency depends on usage intensity and terrain conditions. Light residential use might require line replacement every few weeks, while heavy use in rocky areas could necessitate replacement multiple times per season. Monitor line length regularly and reload when it becomes noticeably shorter.

Q: Can I use line diameter sizes other than .080 and .095 millimeters?

A: No, the Stihl FS 45 is specifically designed for these two diameter sizes. Using incorrect line sizes can affect cutting performance, engine load, and safety. Always refer to your user manual for approved specifications.

Q: What should I do if my trimmer line keeps breaking during use?

A: Frequent line breakage can indicate improper winding tension, striking hard objects repeatedly, or dull blade edges that strain the line. Ensure proper installation, avoid obstacles, and inspect the trimmer head for damage. Consider upgrading to the thicker .095 millimeter line if you frequently encounter tough conditions.

Q: Is it possible to reload the line without removing the trimmer head?

A: Yes, the AutoCut C 6-2 design allows line replacement without complete head removal in most cases. However, if the line is stuck or won’t feed through, you may need to partially disassemble the head as described in the troubleshooting section.

Q: How do I know if my line is wound too tightly?

A: If the white inner ring feels excessively stiff or difficult to rotate, or if the trimmer head vibrates noticeably during operation, the line may be wound too tightly. Rewind it with slightly less tension for smoother operation.