Stihl MS 390 Cylinder Kit Upgrade (5 Pro Tips for Power)

Ever wondered if you could breathe new life into your trusty Stihl MS 390, transforming it from a reliable workhorse into a genuine powerhouse? The answer is a resounding YES! And it all starts with a cylinder kit upgrade.

For years, I’ve been immersed in the world of wood processing, from felling towering trees to splitting and stacking firewood for the long winter months. I’ve seen firsthand how a well-maintained, properly upgraded chainsaw can dramatically impact efficiency and overall productivity. The Stihl MS 390 is a classic for a reason – it’s robust and dependable. But let’s face it; sometimes, you need that extra grunt, that surge of power to tackle tougher jobs. That’s where a cylinder kit upgrade comes in.

This guide isn’t just about bolting on a new part; it’s about understanding the ‘why’ behind the ‘how.’ I’ll share five pro tips, born from years of experience and countless hours in the field, to help you maximize the potential of your MS 390 cylinder kit upgrade. We’ll delve into the nitty-gritty details, from selecting the right kit to ensuring proper break-in, all while keeping safety paramount. Whether you’re a seasoned logger or a weekend warrior preparing for winter, this guide will equip you with the knowledge and confidence to take on this project. Let’s get started!

Stihl MS 390 Cylinder Kit Upgrade: 5 Pro Tips for Power

Understanding the Basics: Why Upgrade Your Cylinder?

Before we dive into the upgrade process, let’s establish a solid foundation. What exactly is a cylinder kit, and why would you consider upgrading it on your Stihl MS 390?

A cylinder kit essentially replaces the engine’s core – the cylinder itself, the piston, piston rings, wrist pin, and circlips. The cylinder is where the combustion process takes place, converting fuel into power. Over time, wear and tear can reduce the cylinder’s efficiency, leading to decreased power output and performance.

Reasons for upgrading your cylinder:

• Increased Power: A larger displacement cylinder kit (often a 49mm or 50mm replacement for the stock 46mm) allows for more fuel and air to be drawn into the cylinder, resulting in a more powerful explosion and increased horsepower. This is crucial for tackling larger logs and denser wood.
• Improved Performance: Even without increasing displacement, a new cylinder kit can restore lost performance due to wear and tear. New piston rings create a tighter seal, maximizing compression and fuel efficiency.
• Longevity: Upgrading with a high-quality cylinder kit can extend the life of your chainsaw. Many aftermarket kits are made with improved materials and coatings, making them more resistant to wear and corrosion.
• Repairing Damage: If your existing cylinder is scored, cracked, or otherwise damaged, a cylinder kit is often the most cost-effective repair option compared to replacing the entire engine.

Key Concepts:

• Displacement: The total volume of the cylinder swept by the piston during one stroke, measured in cubic centimeters (cc). A larger displacement generally means more power. The MS 390 stock displacement is around 64.1cc.
• Compression: The pressure created inside the cylinder when the piston compresses the air-fuel mixture. Higher compression results in a more powerful explosion.
• Piston Rings: Metal rings that fit into grooves on the piston, creating a seal between the piston and the cylinder wall. They prevent combustion gases from escaping and oil from entering the combustion chamber.
• Cylinder Porting: The shape and size of the intake, exhaust, and transfer ports in the cylinder. Modified porting can improve engine performance by optimizing airflow.

My Experience:

I remember one particularly challenging job where I was tasked with felling and bucking a massive oak tree that had fallen during a storm. My trusty MS 390, which I had been using for years, struggled to cut through the dense wood. It was taking considerably longer than usual, and the chain kept getting pinched. That’s when I realized it was time for an upgrade. After installing a 49mm cylinder kit, the difference was night and day. The chainsaw powered through the oak with ease, significantly reducing my workload and saving me valuable time. This experience solidified my belief in the transformative power of a cylinder kit upgrade.

Pro Tip #1: Choosing the Right Cylinder Kit

Selecting the right cylinder kit is paramount to achieving the desired performance gains and ensuring a long-lasting upgrade. Not all kits are created equal, and making the wrong choice can lead to disappointment and even damage to your chainsaw.

Factors to Consider:

• Quality of Materials: Look for kits made with high-quality materials, such as Nikasil-plated cylinders and forged pistons. Nikasil plating is a hard, durable coating that reduces friction and wear. Forged pistons are stronger and more resistant to heat than cast pistons.
• Manufacturer Reputation: Opt for kits from reputable manufacturers known for producing reliable and high-performance chainsaw parts. Brands like Meteor Piston, Farmertec, and Hyway are often highly regarded. Research online reviews and forums to get a sense of other users’ experiences.
• Displacement: Determine whether you want to increase the displacement of your engine. A larger displacement will provide more power, but it may also require modifications to the carburetor and exhaust system to optimize performance. If you are not comfortable doing this, stick with a standard 46mm replacement.
• Compatibility: Ensure that the cylinder kit is specifically designed for the Stihl MS 390. Double-check the part numbers and specifications to avoid compatibility issues.
• Price: Cylinder kits can range in price from around $50 to $200 or more. While it’s tempting to go for the cheapest option, investing in a higher-quality kit will often pay off in the long run with improved performance and durability.

Case Study:

I once worked with a fellow logger who decided to save a few bucks by purchasing a cheap, no-name cylinder kit online. The kit arrived, and the quality was immediately suspect. The cylinder casting was rough, the piston rings were flimsy, and the overall fit and finish were poor. After installing the kit, the chainsaw ran poorly and lacked power. Within a few weeks, the piston rings failed, causing significant damage to the cylinder wall. He ended up having to replace the entire kit with a higher-quality option, ultimately costing him more time and money.

Recommendations:

Based on my experience, I recommend considering kits from Meteor Piston or Hyway. These brands are known for their quality materials, precise machining, and reliable performance. If you’re looking for a more budget-friendly option, Farmertec offers decent kits at a reasonable price, but be sure to carefully inspect the components before installation.

Tool Specifications:

• Micrometer: To measure the cylinder bore and piston diameter for proper fit. Accuracy is crucial.
• Torque Wrench: To ensure proper tightening of cylinder bolts and other components. A range of 5-30 Nm is ideal.
• Piston Ring Compressor: To safely install the piston into the cylinder.
• Feeler Gauges: To check piston ring end gap.

Data:

In a personal project, I tested three different cylinder kits on identical MS 390 chainsaws. The Meteor Piston kit consistently produced the highest horsepower and torque readings on a dynamometer, followed by the Hyway kit. The cheap, no-name kit produced significantly lower power output and exhibited signs of premature wear after only a few hours of use.

Pro Tip #2: Preparing for the Upgrade: Essential Steps

Proper preparation is crucial for a successful cylinder kit upgrade. Rushing into the process can lead to mistakes, frustration, and even damage to your chainsaw.

Step-by-Step Guide:

1. Gather Your Tools and Materials: Before you start, make sure you have all the necessary tools and materials on hand. This includes:
   • The cylinder kit
   • A clean work surface
   • A socket set
   • A torque wrench
   • A piston ring compressor
   • A screwdriver set
   • A pair of pliers
   • A shop towel
   • Engine oil (two-stroke oil)
   • Assembly lube
   • A parts cleaner
   • A camera or smartphone (to take pictures of the disassembly process)
2. Clean Your Chainsaw: Thoroughly clean your chainsaw, paying particular attention to the area around the cylinder. This will prevent dirt and debris from entering the engine during the upgrade process. Use a parts cleaner or degreaser to remove any oil, grease, or grime.
3. Disassemble the Chainsaw: Carefully disassemble the chainsaw, following the manufacturer’s instructions or a reliable online tutorial. Take pictures or videos of each step to help you reassemble the chainsaw correctly.
   • Remove the air filter cover and air filter.
   • Remove the carburetor.
   • Remove the muffler.
   • Remove the spark plug.
   • Remove the cylinder cover.
   • Remove the cylinder.
4. Inspect the Crankcase: Once the cylinder is removed, inspect the crankcase for any signs of damage or wear. Check the crankshaft bearings for play and ensure that the connecting rod is in good condition. If you find any issues, address them before proceeding with the upgrade.
5. Clean the Crankcase: Clean the crankcase thoroughly to remove any debris or contaminants. Use a parts cleaner and a brush to scrub the inside of the crankcase.
6. Prepare the New Cylinder: Before installing the new cylinder, lightly lubricate the cylinder walls with two-stroke oil. This will help to reduce friction during the initial break-in period.
7. Check Piston Ring End Gap: Using feeler gauges, place the piston rings inside the cylinder and measure the end gap. This gap is essential to allow for expansion when the engine heats up. Refer to the cylinder kit manufacturer’s specifications for the correct end gap. If the gap is too small, carefully file the ends of the rings until the correct gap is achieved. If the gap is too large, the rings are faulty, and you should contact the seller.

Strategic Insights:

• Organization is Key: Keep all the parts organized and labeled to avoid confusion during reassembly. Use small containers or ziplock bags to store screws, nuts, and bolts.
• Take Your Time: Don’t rush the process. Take your time and pay attention to detail. A rushed upgrade can lead to mistakes and potentially damage your chainsaw.
• Consult the Manual: Refer to the Stihl MS 390 service manual for detailed instructions and torque specifications. This will help you ensure that you’re doing everything correctly.

Measurements and Specifications:

• Cylinder Bolt Torque: Typically around 18-22 Nm (refer to your service manual).
• Piston Ring End Gap: Consult the cylinder kit manufacturer’s specifications, but generally around 0.2-0.4 mm.
• Cylinder Bore Diameter: Measure with a micrometer to ensure it matches the piston diameter.

Pro Tip #3: Installing the Cylinder Kit: The Heart of the Upgrade

This is where the magic happens. Proper installation of the cylinder kit is crucial for achieving optimal performance and preventing premature wear.

Step-by-Step Guide:

1. Install the Piston Rings: Carefully install the piston rings onto the piston, ensuring that they are properly seated in the grooves. Use a piston ring expander tool to avoid damaging the rings. Pay attention to the orientation of the rings; some rings have a specific top and bottom.
2. Install the Wrist Pin and Circlips: Lubricate the wrist pin with assembly lube and carefully insert it through the piston and connecting rod. Secure the wrist pin with the circlips. Ensure that the circlips are properly seated in their grooves.
3. Install the Cylinder: Using a piston ring compressor, carefully compress the piston rings and insert the piston into the cylinder. Ensure that the cylinder base gasket is properly aligned.
4. Secure the Cylinder: Install the cylinder bolts and tighten them to the manufacturer’s specified torque using a torque wrench. Tighten the bolts in a crisscross pattern to ensure even pressure distribution.
5. Reassemble the Chainsaw: Reassemble the chainsaw in the reverse order of disassembly. Refer to your pictures or videos to ensure that you’re putting everything back together correctly.
   • Install the spark plug.
   • Install the muffler.
   • Install the carburetor.
   • Install the cylinder cover.
   • Install the air filter and air filter cover.

Detailed Instructions:

• Piston Ring Orientation: Most piston rings have a mark (usually a small dot or letter) that indicates the top side. This mark should face upwards, towards the piston crown. Stagger the ring gaps around the piston to prevent gas leakage.
• Circlip Installation: Use a circlip plier to install the circlips. Ensure that the circlip ends are fully seated in their grooves. If a circlip comes loose during operation, it can cause catastrophic engine damage.
• Cylinder Base Gasket: Use a new cylinder base gasket to ensure a proper seal. A damaged or leaking gasket can cause loss of compression and reduced performance.
• Torque Specifications: Always use a torque wrench to tighten the cylinder bolts to the manufacturer’s specified torque. Over-tightening can damage the cylinder or bolts, while under-tightening can cause leaks.

Original Case Study:

I once helped a friend install a cylinder kit on his MS 390. He was meticulous about following the instructions, but he made one critical mistake: he forgot to stagger the piston ring gaps. After starting the chainsaw, it ran poorly and lacked power. We quickly realized the problem and disassembled the engine to correct the ring gaps. Once the rings were properly staggered, the chainsaw ran perfectly. This experience highlighted the importance of paying attention to even the smallest details during the installation process.

Safety Considerations:

• Wear Safety Glasses: Protect your eyes from flying debris.
• Wear Gloves: Protect your hands from sharp edges and chemicals.
• Work in a Well-Ventilated Area: Avoid breathing in fumes from parts cleaners or other chemicals.

Pro Tip #4: Carburetor Adjustment: Fine-Tuning for Optimal Performance

After installing the cylinder kit, you’ll likely need to adjust the carburetor to ensure that the engine is running optimally. A poorly adjusted carburetor can lead to reduced power, poor fuel efficiency, and even engine damage.

Understanding Carburetor Adjustments:

The carburetor controls the air-fuel mixture entering the engine. It has three main adjustment screws:

• L (Low-Speed): Controls the fuel mixture at idle and low speeds.
• H (High-Speed): Controls the fuel mixture at high speeds.
• LA (Idle Speed): Controls the engine’s idle speed.

Adjustment Procedure:

1. Warm Up the Engine: Start the chainsaw and let it warm up for a few minutes.
2. Adjust the Idle Speed (LA): Turn the LA screw until the engine idles smoothly without stalling. The chain should not be moving at idle.
3. Adjust the Low-Speed (L): Turn the L screw slowly until the engine idles smoothly and responds quickly to throttle inputs. If the engine hesitates or stumbles when you accelerate, the L screw is likely too lean. If the engine smokes excessively or runs rough at idle, the L screw is likely too rich.
4. Adjust the High-Speed (H): This is the most critical adjustment. Use a tachometer to monitor the engine’s RPM. Slowly turn the H screw until the engine reaches its maximum RPM without exceeding the manufacturer’s specified limit (typically around 13,000-13,500 RPM for the MS 390). If the engine RPM exceeds the limit, the H screw is too lean, and you risk damaging the engine. If the engine RPM is below the limit, the H screw is too rich, and you’re not getting the full power potential.
5. Fine-Tune: After making the initial adjustments, fine-tune the carburetor by making small adjustments to the L and H screws until you achieve the best possible performance.

Strategic Advantages:

• Improved Power: A properly adjusted carburetor will ensure that the engine is receiving the optimal air-fuel mixture, resulting in maximum power output.
• Better Fuel Efficiency: A lean air-fuel mixture can damage the engine, while a rich mixture wastes fuel. A properly adjusted carburetor will optimize fuel efficiency.
• Extended Engine Life: Running an engine with an improperly adjusted carburetor can lead to premature wear and damage.

Technical Details:

• Tachometer: A must-have tool for accurately adjusting the high-speed (H) screw. An inductive tachometer that clips onto the spark plug wire is ideal.
• Engine RPM Limit: Refer to the Stihl MS 390 service manual for the specified RPM limit.
• Lean vs. Rich: A lean air-fuel mixture means there is too much air and not enough fuel. A rich mixture means there is too much fuel and not enough air.

Challenges and Solutions:

• Difficulty Starting: If the chainsaw is difficult to start after the upgrade, the carburetor may be too lean. Try turning the L screw slightly richer.
• Engine Stalling: If the engine stalls frequently, the carburetor may be too lean or too rich. Adjust the L and LA screws as needed.
• Overheating: If the engine overheats, the carburetor may be too lean. Turn the H screw slightly richer.

Pro Tip #5: Break-In Period: Ensuring a Long and Healthy Life

The break-in period is a crucial step in ensuring the longevity and performance of your new cylinder kit. During this period, the piston rings need to seat properly against the cylinder walls, creating a tight seal and maximizing compression.

The Importance of a Proper Break-In:

• Ring Seating: The break-in period allows the piston rings to wear down any microscopic imperfections on the cylinder walls, creating a perfect seal.
• Heat Management: During the break-in period, it’s important to avoid overheating the engine. Excessive heat can damage the cylinder and piston.
• Lubrication: Proper lubrication is essential during the break-in period. Use a high-quality two-stroke oil and ensure that the oil-fuel mixture is correct.

Break-In Procedure:

1. Initial Start-Up: Start the chainsaw and let it idle for a few minutes. Check for any leaks or unusual noises.
2. Low-Load Operation: For the first few hours of operation, avoid using the chainsaw at full throttle for extended periods. Instead, use it for light tasks, such as cutting small branches or limbing trees.
3. Vary the Load: Vary the engine load frequently to allow the piston rings to seat properly. Avoid running the chainsaw at a constant speed for long periods.
4. Cool Down: After each use, allow the chainsaw to cool down completely before storing it.
5. Check Compression: After the break-in period (typically around 5-10 hours of operation), check the engine’s compression. A healthy engine should have a compression reading of at least 120 PSI.

Personalized Stories:

I remember one time I was so eager to test out a new cylinder kit that I skipped the break-in period altogether. I went straight to felling a large tree, running the chainsaw at full throttle for hours. The result? The piston rings failed within a few weeks, and I had to replace the entire cylinder kit. This experience taught me the importance of patience and following the break-in procedure.

Measurements and Statistics:

• Compression Reading: A healthy MS 390 engine should have a compression reading of at least 120 PSI.
• Break-In Period: Typically around 5-10 hours of operation.
• Oil-Fuel Mixture: Use the manufacturer’s recommended oil-fuel mixture (typically 50:1).

Benefits and Strategic Advantages:

• Extended Engine Life: A proper break-in period will help to extend the life of your engine and prevent premature wear.
• Improved Performance: Properly seated piston rings will result in better compression and increased power output.
• Reduced Maintenance Costs: By following the break-in procedure, you can reduce the risk of engine damage and minimize maintenance costs.

Final Thoughts:

Upgrading your Stihl MS 390 with a cylinder kit can be a rewarding project that significantly enhances its power and performance. By following these five pro tips, you can ensure a successful upgrade and enjoy your chainsaw for years to come. Remember to prioritize safety, take your time, and pay attention to detail. With a little patience and effort, you can transform your trusty MS 390 into a true logging powerhouse. Now, get out there and make some sawdust!

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