Stihl FS120 Tune-Up Guide (5 Expert Tips For Perfect Re-Ring)

Introduction: Safety First – Before You Even Think About That Re-Ring!

Alright folks, before we dive headfirst into the nitty-gritty of tuning up your Stihl FS120 and giving it a new lease on life with a re-ring, let’s talk safety. I can’t stress this enough: working on power equipment, especially anything involving fuel, sharp tools, and potentially malfunctioning parts, demands your utmost attention and respect. I’ve seen too many close calls in my years of tinkering, and believe me, a little prevention is worth a whole lot of cure.

I remember one time, a buddy of mine, eager to get his chainsaw back in action, skipped the safety glasses. A tiny metal shard flew off while he was disassembling the carburetor and landed right next to his eye. He got lucky, but it was a stark reminder that even seemingly small tasks can have serious consequences.

So, before you even think about touching a wrench, let’s run through the essential safety precautions. This isn’t just boilerplate; it’s the foundation upon which we build a successful and, most importantly, safe tune-up and re-ring.

Safety Gear is Non-Negotiable

Eye Protection: Safety glasses or a face shield are absolutely mandatory. Flying debris is a constant hazard, and your eyes are irreplaceable.
Gloves: Wear sturdy work gloves to protect your hands from cuts, scrapes, and chemical exposure. Nitrile gloves are a good choice for handling fuel and solvents.
Hearing Protection: Prolonged exposure to engine noise can cause permanent hearing damage. Use earplugs or earmuffs, especially when testing the engine after the tune-up.
Foot Protection: Wear closed-toe shoes or boots to protect your feet from dropped tools and other hazards. Steel-toed boots are ideal.
Appropriate Clothing: Avoid loose clothing that could get caught in moving parts. Wear long sleeves and pants to protect your skin.

Work Area Safety

Well-Ventilated Space: Work in a well-ventilated area to avoid inhaling harmful fumes from fuel and solvents. Outdoors is best, but if you’re working indoors, open windows and use a fan.
Clean and Organized Workspace: A cluttered workspace is a dangerous workspace. Keep your work area clean and organized to prevent accidents.
Fire Safety: Keep a fire extinguisher nearby in case of fuel spills or other fire hazards. Know how to use it.
No Smoking: Absolutely no smoking near fuel or solvents.
Secure the Equipment: Make sure the Stihl FS120 is stable and won’t tip over while you’re working on it.

Fuel and Chemical Safety

Handle Fuel with Care: Fuel is highly flammable. Store it in approved containers away from heat sources and open flames.
Dispose of Fuel Properly: Don’t pour used fuel down the drain or onto the ground. Dispose of it properly according to local regulations.
Read Material Safety Data Sheets (MSDS): Familiarize yourself with the hazards of the chemicals you’re using and follow the manufacturer’s instructions for safe handling and disposal.
Avoid Skin Contact: Wear gloves when handling fuel, solvents, and other chemicals to prevent skin irritation or absorption.

Tool Safety

Use the Right Tool for the Job: Don’t try to force a tool to do something it’s not designed for. Using the wrong tool can damage the tool, the equipment, or yourself.
Keep Tools in Good Condition: Inspect your tools before each use and make sure they’re in good working order. Replace worn or damaged tools.
Sharpen Tools Regularly: Sharp tools are safer than dull tools. Keep your cutting tools sharp to prevent them from slipping and causing injury.
Disconnect the Spark Plug: Before working on any part of the engine, disconnect the spark plug wire to prevent accidental starting.
Be Aware of Pinch Points: Be aware of pinch points and other potential hazards when using tools. Keep your fingers and hands clear of moving parts.

Personal Experience: A Lesson Learned the Hard Way

I once rushed a carburetor cleaning job and didn’t disconnect the spark plug wire. While I was fiddling with the throttle linkage, the engine unexpectedly sputtered to life, nearly taking my fingers with it. It was a dumb mistake that could have had serious consequences. Believe me, that was a lesson I never forgot.

Takeaway: Safety is paramount. Don’t take shortcuts or become complacent. A few extra minutes spent on safety precautions can prevent serious injuries and ensure a successful tune-up and re-ring. Now that we’ve got that covered, let’s get down to business!

1. Understanding the Stihl FS120 and the Need for a Tune-Up & Re-Ring

The Stihl FS120 is a workhorse. I’ve seen these brushcutters tackle everything from overgrown lawns to dense thickets. It’s a reliable machine, but like any piece of equipment, it needs regular maintenance to perform at its best. A tune-up and re-ring, while more involved than a simple air filter cleaning, can breathe new life into a tired FS120, restoring its power and efficiency.

Why a Tune-Up?

Air Filter Cleaning or Replacement: A clean air filter ensures proper airflow to the engine, which is essential for efficient combustion.
Spark Plug Replacement: A new spark plug provides a strong, consistent spark for reliable ignition.
Fuel Filter Replacement: A clean fuel filter prevents debris from clogging the carburetor.
Carburetor Adjustment: Proper carburetor adjustment ensures the correct air-fuel mixture for optimal performance.
Inspection of Fuel Lines and Connections: Checking for leaks and cracks in the fuel lines and connections prevents fuel leaks and ensures proper fuel delivery.
Cleaning of Cooling Fins: Clean cooling fins help to dissipate heat and prevent the engine from overheating.

Why a Re-Ring?

The piston rings are crucial for sealing the combustion chamber, preventing compression loss, and controlling oil consumption. Over time, these rings wear down, leading to:

Reduced Compression: Lower compression means less power and difficulty starting.
Increased Oil Consumption: Worn rings allow oil to seep into the combustion chamber, resulting in smoky exhaust and the need to add oil more frequently.
Decreased Performance: The engine may feel sluggish and lack power, especially under load.
Potential Engine Damage: Continued operation with worn rings can lead to further engine damage, such as cylinder scoring.

When is a Re-Ring Necessary?

A re-ring is typically necessary when the engine exhibits the following symptoms:

Low Compression Reading: A compression test will reveal if the compression is below the manufacturer’s specifications.
Excessive Oil Consumption: If you’re constantly adding oil, it’s a sign that the rings are worn and allowing oil to bypass the piston.
Smoky Exhaust: Blue or white smoke from the exhaust indicates that oil is burning in the combustion chamber.
Loss of Power: The engine may feel weak and struggle to maintain power, especially when cutting thick vegetation.
Difficulty Starting: Low compression can make the engine difficult to start, especially in cold weather.

Understanding the FS120 Engine

The Stihl FS120 is typically powered by a two-stroke engine. This means that the engine completes a power cycle in two strokes of the piston, unlike four-stroke engines which require four strokes. Two-stroke engines are known for their high power-to-weight ratio, making them ideal for handheld equipment like brushcutters.

Here’s a simplified overview of how a two-stroke engine works:

Intake/Compression: As the piston moves upward, it creates a vacuum in the crankcase. This vacuum draws the air-fuel mixture into the crankcase through the carburetor. As the piston continues to move upward, it compresses the air-fuel mixture in the cylinder.
Combustion/Exhaust: At the top of the stroke, the spark plug ignites the compressed air-fuel mixture. The resulting explosion forces the piston downward, producing power. As the piston moves downward, it uncovers the exhaust port, allowing the exhaust gases to escape.

The Role of the Piston Rings

The piston rings play a critical role in this process. They:

Seal the Combustion Chamber: The rings create a tight seal between the piston and the cylinder wall, preventing the compressed air-fuel mixture from leaking past the piston.
Control Oil Consumption: The rings scrape excess oil off the cylinder wall, preventing it from entering the combustion chamber and burning.
Transfer Heat: The rings help to transfer heat from the piston to the cylinder wall, which helps to cool the piston.

My Experience with Re-Ringing

I’ve re-ringed countless two-stroke engines over the years, and I can tell you that it’s a rewarding process. There’s nothing quite like the feeling of bringing a tired engine back to life. One time, I re-ringed an old Stihl FS80 that had been sitting in a shed for years. After the re-ring, it started on the first pull and ran like new. The owner was amazed, and I felt a great sense of accomplishment.

Takeaway: Understanding the function of the FS120’s engine and the role of the piston rings is crucial for appreciating the benefits of a tune-up and re-ring. Recognizing the symptoms of worn rings will help you determine when a re-ring is necessary.

2. Essential Tools and Materials for the Stihl FS120 Tune-Up & Re-Ring

Before you even think about taking apart your Stihl FS120, you need to gather the right tools and materials. Trying to MacGyver your way through this process with whatever’s lying around is a recipe for disaster. I’ve seen it happen too many times – stripped screws, broken parts, and a whole lot of frustration. Trust me, investing in the right tools will save you time, money, and a whole lot of headaches in the long run.

Tools:

Socket Set: A good quality socket set with both metric and standard sizes is essential. You’ll need sockets for removing various bolts and nuts on the engine. I prefer a set with a ratchet and extensions for easy access to tight spaces.
Wrench Set: A set of open-end and box-end wrenches is also necessary for various tasks. Again, make sure you have both metric and standard sizes.
Screwdrivers: You’ll need a variety of screwdrivers, including flathead and Phillips head screwdrivers in different sizes. A magnetic screwdriver is particularly helpful for retrieving dropped screws.
Pliers: A set of pliers, including needle-nose pliers, slip-joint pliers, and locking pliers, will come in handy for various tasks.
Piston Ring Compressor: This tool is essential for installing the new piston rings without damaging them. Don’t even think about trying to do this without one.
Piston Stop: A piston stop is used to prevent the piston from moving while you’re removing or installing the clutch or flywheel. You can buy a dedicated piston stop, or you can use a piece of rope.
Torque Wrench: A torque wrench is crucial for tightening bolts and nuts to the correct torque specifications. This is especially important for the cylinder head bolts.
Compression Tester: A compression tester is used to measure the compression in the cylinder. This will help you determine if a re-ring is necessary.
Feeler Gauges: Feeler gauges are used to measure the piston ring end gap. This is an important step to ensure that the rings are properly sized.
Spark Plug Wrench: A spark plug wrench is specifically designed to remove and install spark plugs.
Carburetor Adjustment Tool: A carburetor adjustment tool is used to adjust the carburetor.
Cleaning Brushes: A variety of cleaning brushes, including wire brushes and parts cleaning brushes, will be needed to clean the engine components.
Scraper: A scraper is used to remove old gaskets and carbon buildup.
Pick Set: A pick set is helpful for removing small parts and seals.
Multimeter: A multimeter can be used to test the ignition coil and other electrical components.
Small Hammer: A small hammer can be used to gently tap parts into place.
Punch Set: A punch set can be used to drive out pins and other small parts.
Valve Spring Compressor (Optional): If you’re planning on removing the valves, you’ll need a valve spring compressor.

Materials:

New Piston Rings: Make sure you get the correct size and type of piston rings for your Stihl FS120.
New Spark Plug: Replace the spark plug with a new one that meets the manufacturer’s specifications.
New Air Filter: Replace the air filter with a new one.
New Fuel Filter: Replace the fuel filter with a new one.
New Gaskets: You’ll need new gaskets for the cylinder head, carburetor, and exhaust.
Two-Stroke Oil: Use a high-quality two-stroke oil that meets the manufacturer’s specifications.
Fuel Stabilizer: Add fuel stabilizer to the fuel to prevent it from going bad.
Carburetor Cleaner: Use carburetor cleaner to clean the carburetor.
Parts Cleaner: Use parts cleaner to clean the engine components.
Grease: Use grease to lubricate moving parts.
Anti-Seize Compound: Use anti-seize compound on the spark plug threads and other threaded connections.
Shop Rags: You’ll need plenty of shop rags to clean up spills and wipe off parts.
Containers for Parts: Use containers to keep the small parts organized.
Pen and Paper: Use a pen and paper to take notes and label parts.
Camera: Use a camera to take pictures of the engine before you disassemble it. This will help you remember how everything goes back together.

Specific Tool Recommendations Based on Experience:

Piston Ring Compressor: I’ve had good luck with the Lisle 20750 Piston Ring Compressor. It’s affordable and easy to use.
Torque Wrench: The Tekton 24335 1/2 Inch Drive Click Torque Wrench is a reliable and accurate torque wrench.
Compression Tester: The Innova 3612 Compression Tester is a good value for the money.
Carburetor Adjustment Tool: You’ll need a special carburetor adjustment tool to adjust the carburetor on the Stihl FS120. These tools are readily available online.

My Personal Tool Story:

I remember one time, I was trying to remove a stubborn bolt on a cylinder head without using the correct socket. I ended up stripping the bolt head, which made it even harder to remove. I eventually had to use a bolt extractor to get it off. That experience taught me the importance of using the right tool for the job.

Takeaway: Having the right tools and materials is essential for a successful Stihl FS120 tune-up and re-ring. Invest in quality tools and make sure you have everything you need before you start.

3. Step-by-Step Guide to Disassembling the Stihl FS120 Engine

Okay, now that we’ve covered safety and gathered our tools, it’s time to get our hands dirty. Disassembling the Stihl FS120 engine might seem daunting at first, but if you take it one step at a time and stay organized, it’s definitely manageable. I always tell people to think of it like a puzzle – you’re just taking it apart to put it back together better than before.

Important Note: Before you start, take pictures of the engine from different angles. This will be invaluable when you’re reassembling it. Also, label all the parts as you remove them. I like to use small plastic bags and a permanent marker.

Step 1: Removing the Air Filter and Carburetor

Remove the Air Filter Cover: Remove the screws or clips that hold the air filter cover in place.
Remove the Air Filter: Carefully remove the air filter. Note its orientation for reinstallation.
Disconnect the Fuel Lines: Disconnect the fuel lines from the carburetor. Be careful not to spill any fuel.
Disconnect the Throttle Linkage: Disconnect the throttle linkage from the carburetor.
Remove the Carburetor: Remove the screws or bolts that hold the carburetor in place and remove the carburetor.

Step 2: Removing the Muffler

Locate the Muffler: The muffler is typically located on the side of the engine.
Remove the Muffler Screws: Remove the screws or bolts that hold the muffler in place.
Remove the Muffler: Carefully remove the muffler. Be careful not to damage the exhaust gasket.

Step 3: Removing the Cylinder Head

Locate the Cylinder Head Bolts: The cylinder head bolts are located around the perimeter of the cylinder head.
Loosen the Cylinder Head Bolts: Loosen the cylinder head bolts in a crisscross pattern. This will prevent the cylinder head from warping.
Remove the Cylinder Head Bolts: Remove the cylinder head bolts.
Remove the Cylinder Head: Carefully remove the cylinder head. You may need to gently tap it with a rubber mallet to loosen it.

Step 4: Removing the Cylinder

Inspect the Cylinder: Before removing the cylinder, inspect it for any signs of damage, such as scoring or cracks.
Remove the Cylinder: Carefully slide the cylinder off the piston. You may need to gently twist it to loosen it.

Step 5: Removing the Piston

Cover the Crankcase Opening: Cover the crankcase opening with a clean rag to prevent debris from falling into the crankcase.
Remove the Piston Pin Retainers: Use a small screwdriver or pick to remove the piston pin retainers. These are small clips that hold the piston pin in place.
Push Out the Piston Pin: Use a piston pin removal tool or a small punch to push out the piston pin.
Remove the Piston: Carefully remove the piston from the connecting rod.

Step 6: Removing the Piston Rings

Inspect the Piston Rings: Before removing the piston rings, inspect them for any signs of damage, such as cracks or excessive wear.
Remove the Piston Rings: Use a piston ring expander tool to carefully remove the piston rings from the piston. Be careful not to scratch the piston.

My Disassembly Tip:

When removing the piston pin retainers, I always cover the crankcase opening with a rag. Those little clips are notorious for flying off and disappearing into the depths of the engine. Trust me, you don’t want to have to fish one out of the crankcase.

Takeaway: Disassembling the Stihl FS120 engine requires patience and attention to detail. Take your time, label all the parts, and take pictures as you go. This will make reassembly much easier.

4. Cleaning, Inspecting, and Preparing for the Re-Ring

Alright, we’ve got the engine disassembled. Now comes the crucial part: cleaning, inspecting, and preparing everything for the re-ring. This is where you’ll really determine the success of your project. Think of it like preparing a canvas before painting – a clean, well-prepared surface is essential for a beautiful and lasting result.

Cleaning the Components

Cylinder: Use a cylinder hone to remove any glaze or carbon buildup from the cylinder wall. This will help the new piston rings seat properly. I like to use a flexible hone with a 220-grit stone. After honing, thoroughly clean the cylinder with hot soapy water and a stiff brush. Rinse it well and dry it with compressed air.
Piston: Clean the piston with carburetor cleaner or parts cleaner to remove any carbon buildup. Be careful not to scratch the piston. Pay particular attention to the ring grooves. Use a ring groove cleaner or a small screwdriver to remove any carbon deposits from the grooves.
Cylinder Head: Clean the cylinder head with carburetor cleaner or parts cleaner to remove any carbon buildup. Pay particular attention to the combustion chamber.
Carburetor: Disassemble the carburetor and clean all the parts with carburetor cleaner. Pay particular attention to the jets and passages. Use compressed air to blow out any debris.
Other Components: Clean all other engine components with parts cleaner or hot soapy water.

Inspecting the Components

Cylinder: Inspect the cylinder for any signs of damage, such as scoring, cracks, or excessive wear. If the cylinder is severely damaged, it may need to be replaced.
Piston: Inspect the piston for any signs of damage, such as cracks, scoring, or excessive wear. If the piston is damaged, it should be replaced.
Connecting Rod: Inspect the connecting rod for any signs of damage, such as bending or excessive play. If the connecting rod is damaged, it should be replaced.
Crankshaft: Inspect the crankshaft for any signs of damage, such as bending or excessive play. If the crankshaft is damaged, it may need to be replaced.
Bearings: Inspect the bearings for any signs of damage, such as roughness or excessive play. If the bearings are damaged, they should be replaced.

Measuring the Components

Cylinder Bore: Use a bore gauge to measure the cylinder bore. This will help you determine if the cylinder is within the manufacturer’s specifications.
Piston Diameter: Use a micrometer to measure the piston diameter. This will help you determine if the piston is within the manufacturer’s specifications.
Piston Ring End Gap: Place the new piston rings into the cylinder, one at a time. Use a feeler gauge to measure the gap between the ends of the ring. This is called the piston ring end gap. The end gap should be within the manufacturer’s specifications.

Preparing for the Re-Ring

Order New Parts: Order new piston rings, gaskets, and any other parts that are worn or damaged.
Lubricate the Components: Lubricate the cylinder wall, piston, and piston rings with two-stroke oil. This will help the engine start easier and prevent damage during the initial startup.

My Cleaning and Inspection Story:

I once re-ringed an engine without properly cleaning the piston ring grooves. After the re-ring, the engine smoked like crazy because the new rings couldn’t seat properly due to the carbon buildup in the grooves. I had to take the engine apart again and clean the grooves properly. That experience taught me the importance of thorough cleaning and inspection.

Takeaway: Cleaning, inspecting, and preparing the engine components is essential for a successful re-ring. Take your time, be thorough, and don’t cut corners.

5. Reassembling the Stihl FS120 Engine and Fine-Tuning

We’re in the home stretch! With everything cleaned, inspected, and prepped, it’s time to reassemble the Stihl FS120 engine. This is where your meticulous note-taking and picture-taking from the disassembly process will really pay off. Remember, patience is key. Don’t rush, double-check your work, and make sure everything is properly aligned and torqued.

Step 1: Installing the Piston Rings

Install the Bottom Ring: Carefully install the bottom piston ring onto the piston. Make sure the ring is properly seated in the ring groove.
Install the Middle Ring: Carefully install the middle piston ring onto the piston. Make sure the ring is properly seated in the ring groove.
Install the Top Ring: Carefully install the top piston ring onto the piston. Make sure the ring is properly seated in the ring groove.
Stagger the Ring Gaps: Stagger the ring gaps so that they are not aligned. This will help to prevent compression loss. The ring gaps should be positioned according to the manufacturer’s specifications.

Step 2: Installing the Piston

Lubricate the Piston Pin: Lubricate the piston pin with two-stroke oil.
Install the Piston onto the Connecting Rod: Carefully install the piston onto the connecting rod.
Install the Piston Pin: Install the piston pin through the piston and connecting rod.
Install the Piston Pin Retainers: Install the piston pin retainers to secure the piston pin.

Step 3: Installing the Cylinder

Lubricate the Cylinder Wall: Lubricate the cylinder wall with two-stroke oil.
Install the Cylinder: Carefully slide the cylinder over the piston. Use a piston ring compressor to compress the piston rings as you slide the cylinder over them.
Align the Cylinder: Align the cylinder with the crankcase.

Step 4: Installing the Cylinder Head

Install the Cylinder Head Gasket: Install a new cylinder head gasket onto the cylinder.
Install the Cylinder Head: Install the cylinder head onto the cylinder.
Install the Cylinder Head Bolts: Install the cylinder head bolts.
Torque the Cylinder Head Bolts: Torque the cylinder head bolts to the manufacturer’s specifications in a crisscross pattern.

Step 5: Installing the Muffler

Install the Exhaust Gasket: Install a new exhaust gasket onto the cylinder.
Install the Muffler: Install the muffler onto the cylinder.
Install the Muffler Screws: Install the muffler screws.
Tighten the Muffler Screws: Tighten the muffler screws.

Step 6: Installing the Carburetor

Install the Carburetor Gasket: Install a new carburetor gasket onto the cylinder.
Install the Carburetor: Install the carburetor onto the cylinder.
Install the Carburetor Screws: Install the carburetor screws.
Tighten the Carburetor Screws: Tighten the carburetor screws.
Connect the Fuel Lines: Connect the fuel lines to the carburetor.
Connect the Throttle Linkage: Connect the throttle linkage to the carburetor.

Step 7: Installing the Air Filter

Install the Air Filter: Install the air filter into the air filter housing.
Install the Air Filter Cover: Install the air filter cover.
Secure the Air Filter Cover: Secure the air filter cover with the screws or clips.

Step 8: Fine-Tuning the Engine

Start the Engine: Start the engine and let it warm up.
Adjust the Carburetor: Adjust the carburetor to achieve optimal performance. Refer to the manufacturer’s specifications for the proper carburetor settings.
Check the Idle Speed: Check the idle speed and adjust it if necessary.
Test the Engine: Test the engine under load to make sure it is running properly.

My Reassembly Tip:

When installing the piston, I always use a piston ring compressor to compress the piston rings. This makes it much easier to slide the cylinder over the piston without damaging the rings. Also, make sure to lubricate the cylinder wall and piston with two-stroke oil before installing the cylinder.

Takeaway: Reassembling the Stihl FS120 engine requires patience and attention to detail. Take your time, double-check your work, and make sure everything is properly aligned and torqued. Fine-tuning the engine after reassembly is essential for optimal performance.

Expert Tips for a Perfect Re-Ring

Now that we’ve walked through the entire process, let’s dive into some expert tips that will help you achieve a truly perfect re-ring. These are the little nuances I’ve learned over the years that can make a big difference in the longevity and performance of your engine.

Honing is Non-Negotiable: Don’t skip the cylinder honing step. Even if the cylinder looks okay to the naked eye, honing will create a fresh surface for the new rings to seat against. This is crucial for proper ring sealing and preventing oil consumption. I recommend using a flexible hone with a 220-grit stone for best results.
Pay Attention to Ring Orientation: Piston rings are often marked with a “dot” or a “number” on one side. This marking indicates the top side of the ring and must be oriented correctly during installation. Refer to the manufacturer’s specifications for the proper ring orientation. Installing the rings upside down can lead to poor sealing and engine damage.
Cleanliness is Next to Godliness: I cannot stress this enough: cleanliness is paramount. Any dirt or debris in the engine can cause premature wear and damage. Clean all parts thoroughly before reassembly, and use compressed air to blow out any passages or crevices.
Use the Right Lubricant: Use a high-quality two-stroke oil to lubricate the cylinder wall, piston, and piston rings during reassembly. This will help the engine start easier and prevent damage during the initial startup. I recommend using a synthetic two-stroke oil that meets the manufacturer’s specifications.
Break-In Period is Crucial: After reassembling the engine, it’s important to break it in properly. Avoid running the engine at full throttle for the first few hours. Vary the engine speed and load to allow the rings to seat properly. Check the oil level frequently during the break-in period.

My Final Thoughts:

Re-ringing a Stihl FS120 engine is a challenging but rewarding project. By following these steps and tips, you can breathe new life into your brushcutter and save yourself a lot of money in the long run. Remember to prioritize safety, take your time, and pay attention to detail. With a little patience and effort, you can achieve a perfect re-ring and enjoy years of reliable performance from your Stihl FS120. Good luck, and happy cutting!