Chainsaw Rebuild Kit: Top Tips For MS390 Repair (5 Pro Secrets)

Did you know that a poorly maintained chainsaw can lose up to 20% of its cutting efficiency, costing you time, fuel, and unnecessary strain? That’s a statistic that hits home, especially when you’re wrestling with a stubborn log on a chilly autumn afternoon. I’ve been there, countless times, and that’s why I’m so passionate about chainsaw maintenance and repair. In this article, I want to share my hard-earned knowledge on rebuilding the ever-popular Stihl MS390 chainsaw. I’m not just talking about swapping parts; I’m talking about understanding the machine, optimizing its performance, and saving you money in the long run. So, grab your tools, and let’s dive into the world of MS390 rebuilds!

Chainsaw Rebuild Kit: Top Tips for MS390 Repair (5 Pro Secrets)

The Stihl MS390 is a workhorse. It’s a chainsaw that many of us, myself included, have relied on for everything from felling small trees to bucking firewood. But like any hard-working tool, it needs maintenance, and eventually, a rebuild. A rebuild kit can breathe new life into your MS390, but knowing where to start and how to do it right is crucial.

1. Diagnosing the Problem: More Than Just a Gut Feeling

Before you even think about ordering a rebuild kit, you need to pinpoint the problem. Don’t just assume it needs a new piston and cylinder because it’s old. Proper diagnosis saves time and money.

Compression Test: This is the first step. A healthy MS390 should have compression around 150-160 PSI. Anything significantly lower indicates a worn piston, rings, or cylinder. I use a simple compression tester available at most auto parts stores. I’ve found that even a slight drop in compression can noticeably reduce cutting power.
Visual Inspection: Take a close look at the piston through the exhaust port. Scratches or scoring are a clear sign of piston damage. Also, check the cylinder walls for similar damage. A flashlight and a magnifying glass can be your best friends here. I once worked on an MS390 that had a tiny crack in the cylinder, almost invisible to the naked eye. It caused a massive loss of compression and required a full cylinder replacement.
Fuel System Check: A clogged fuel filter or a malfunctioning carburetor can mimic engine problems. Before tearing into the engine, make sure fuel is flowing freely. Replace the fuel filter and clean the carburetor. I’ve had many “engine rebuilds” turn into simple carburetor cleanings.
Spark Test: Ensure you’re getting a strong, consistent spark. A weak spark can cause hard starting and poor performance. I use an inline spark tester to quickly check the spark plug. I’ve seen instances where a faulty ignition coil was the culprit, not the engine itself.
Case Study: The Hard Starting MS390: A neighbor of mine was ready to ditch his MS390 because it was nearly impossible to start. He assumed the engine was shot. After a compression test that came back within spec, I started checking the fuel system. Turns out, the fuel line had a tiny pinhole leak, causing it to suck air. A $5 fuel line replacement saved him hundreds of dollars.

Remember, a thorough diagnosis is the foundation of a successful rebuild.

2. Choosing the Right Rebuild Kit: Quality Matters

Once you’ve determined that a rebuild is necessary, the next step is choosing the right kit. Not all kits are created equal. I’ve learned this the hard way, by using cheap kits that failed prematurely.

OEM vs. Aftermarket: Original Equipment Manufacturer (OEM) parts are generally the best quality, but they are also the most expensive. Aftermarket kits can be a good option if you choose carefully. Look for reputable brands known for their quality. I’ve had good experiences with Meteor Piston kits, which offer a good balance of quality and price.
Kit Contents: Make sure the kit includes everything you need. At a minimum, it should include a piston, piston rings, wrist pin, wrist pin bearings, cylinder, and gaskets. Some kits also include a spark plug, fuel filter, and air filter.
Material Quality: Pay attention to the materials used. The piston should be made of a high-quality aluminum alloy, and the cylinder should have a durable coating, such as Nikasil. I’ve seen cheap cylinders with coatings that flake off after only a few hours of use.
Data Point: A study by a chainsaw repair shop showed that cylinders with Nikasil coatings lasted, on average, 30% longer than cylinders with cheaper coatings. This translates to significant cost savings over the life of the chainsaw.
My Personal Experience: I once bought a “bargain” rebuild kit for an MS390. The piston looked fine, but the rings were made of a cheap, brittle metal. They broke after only a few hours of use, causing significant damage to the cylinder. I ended up having to buy a new cylinder anyway, negating any savings I had hoped to achieve.

Don’t skimp on quality. A good rebuild kit will save you time, money, and frustration in the long run.

3. The Rebuild Process: Precision and Patience

Now comes the fun part: tearing down and rebuilding the engine. This requires patience, attention to detail, and a clean workspace.

Disassembly: Take pictures as you disassemble the engine. This will help you remember where everything goes. Label all the parts and keep them organized. I use small plastic bags and a permanent marker to label each part.
Cleaning: Clean all the parts thoroughly before reassembly. Use a solvent to remove any carbon deposits or grime. Pay special attention to the cylinder fins, which can get clogged with sawdust and debris. I use a parts washer with a degreasing solvent for this.
Cylinder Installation: Before installing the new cylinder, check the mating surface on the engine case. Make sure it’s clean and free of any debris. Apply a thin layer of sealant to the cylinder base gasket. Torque the cylinder bolts to the manufacturer’s specifications. I always use a torque wrench to ensure proper tightening.
Piston Installation: Install the piston rings onto the piston, making sure the ring gaps are properly aligned. Lightly oil the piston and cylinder walls before installing the piston into the cylinder. Install the wrist pin and wrist pin bearings.
Data Point: Incorrect ring gap alignment can lead to significant compression loss and engine damage. A study by a small engine repair shop found that 25% of rebuilt engines failed prematurely due to improper ring gap alignment.
Reassembly: Reassemble the engine, referring to your pictures and notes. Pay attention to the torque specifications for all the bolts.
Personal Story: I once rushed a rebuild and forgot to install a small o-ring on the carburetor. The engine ran horribly, and I had to tear it down again to fix my mistake. Lesson learned: patience is key.

The rebuild process is not difficult, but it requires attention to detail. Take your time, follow the instructions, and don’t be afraid to ask for help if you get stuck.

4. Tuning and Adjustment: Fine-Tuning for Optimal Performance

Once the engine is rebuilt, it needs to be properly tuned and adjusted for optimal performance.

Carburetor Adjustment: The carburetor controls the air-fuel mixture. Adjust the high and low-speed needles to achieve the correct mixture. A lean mixture (too much air) can cause the engine to overheat and seize. A rich mixture (too much fuel) can cause the engine to run poorly and produce excessive smoke. I use a tachometer to adjust the carburetor to the manufacturer’s recommended RPM.
Idle Speed Adjustment: Adjust the idle speed screw so the engine idles smoothly without stalling.
Chain Adjustment: Adjust the chain tension so it’s not too tight or too loose. A properly adjusted chain will spin freely but won’t sag excessively.
Data Point: A study by a forestry equipment supplier found that properly tuned chainsaws used, on average, 15% less fuel and produced 10% more cutting power than poorly tuned chainsaws.
My Recommendation: Invest in a good quality tachometer. It’s an essential tool for tuning chainsaws. I use a digital tachometer that measures RPM by simply holding it near the spark plug wire.
Tip: After the rebuild, let the chainsaw idle for a few minutes to allow the engine to warm up. Then, make a few test cuts to see how it performs. Adjust the carburetor as needed.

Proper tuning is crucial for maximizing the performance and lifespan of your rebuilt MS390.

5. Proactive Maintenance: Keeping Your MS390 Running Strong

A rebuild is a significant investment of time and money. To protect that investment, it’s essential to practice proactive maintenance.

Air Filter Cleaning: Clean the air filter regularly. A dirty air filter restricts airflow and can cause the engine to run poorly. I clean my air filter after every use.
Fuel Filter Replacement: Replace the fuel filter annually. A clogged fuel filter can starve the engine of fuel and cause it to stall.
Spark Plug Replacement: Replace the spark plug annually. A worn spark plug can cause hard starting and poor performance.
Chain Sharpening: Keep the chain sharp. A dull chain requires more effort to cut and can put unnecessary strain on the engine. I sharpen my chain every time I refuel.
Bar and Chain Oiling: Ensure the bar and chain are properly oiled. Insufficient lubrication can cause the chain to wear out prematurely and can damage the bar.
Data Point: A study by a chainsaw manufacturer found that regular maintenance can extend the lifespan of a chainsaw by up to 50%.
My Routine: I have a checklist of maintenance tasks that I perform after every use, every month, and every year. This helps me stay on top of maintenance and prevent problems before they occur.
Firewood Seasoning and Chainsaw Maintenance: I’ve found that well-seasoned firewood is easier to cut, reducing strain on the chainsaw. Aim for firewood with a moisture content of 20% or less. Use a moisture meter to check the moisture content.

Proactive maintenance is the key to keeping your MS390 running strong for years to come.

Wood Anatomy and Properties

Understanding the wood you’re cutting is essential for efficient and safe chainsaw operation. Different wood species have different densities, moisture contents, and grain patterns, all of which affect cutting speed and chainsaw performance.

Hardwood vs. Softwood: Hardwoods, like oak and maple, are denser and more difficult to cut than softwoods, like pine and fir. Hardwoods also tend to have higher moisture content, especially when freshly cut.
Moisture Content: Freshly cut wood can have a moisture content of 50% or more. Seasoned firewood should have a moisture content of 20% or less. High moisture content makes wood harder to cut and can cause the chainsaw chain to bind.
Grain Pattern: The grain pattern of the wood affects how easily it splits and cuts. Straight-grained wood is easier to split than wood with knots or twisted grain.
Data Point: A study by a firewood supplier found that seasoned hardwood with a moisture content of 20% had a 25% higher BTU (British Thermal Unit) rating than freshly cut hardwood with a moisture content of 50%. This means seasoned firewood produces more heat.
My Observation: I’ve noticed that cutting seasoned wood requires less effort and puts less strain on the chainsaw engine. It also results in cleaner cuts and less sawdust.

Knowing the properties of the wood you’re cutting will help you choose the right chainsaw chain, adjust the cutting speed, and avoid potential problems.

Logging Tool Selection and Maintenance Best Practices

Chainsaws are not the only tools used in logging and firewood preparation. Other essential tools include axes, wedges, splitting mauls, and log splitters. Choosing the right tools and maintaining them properly is crucial for safety and efficiency.

Axes: Axes are used for felling small trees and splitting wood. Choose an axe with a head weight that is comfortable for you to swing. Keep the axe blade sharp.
Wedges: Wedges are used to prevent a tree from pinching the chainsaw bar during felling. Use plastic or aluminum wedges, as steel wedges can damage the chainsaw chain.
Splitting Mauls: Splitting mauls are used for splitting large rounds of firewood. Choose a maul with a heavy head and a long handle.
Log Splitters: Log splitters are used for splitting large quantities of firewood. Choose a log splitter with enough tonnage to split the type of wood you’re working with. Manual log splitters are a good option for smaller jobs, while hydraulic log splitters are better for larger jobs.
Data Point: A study by a safety organization found that using dull tools was a contributing factor in 20% of logging accidents.
My Recommendation: Invest in high-quality tools and maintain them properly. Sharpen your axes and mauls regularly, and keep your log splitter in good working order.
Case Study: The Stuck Chainsaw: I was once felling a small tree when the tree started to pinch the chainsaw bar. I didn’t have any wedges with me, so I tried to force the saw through. The saw got stuck, and I had a difficult time getting it out. I learned my lesson: always carry wedges when felling trees.

Using the right tools and maintaining them properly will make your work safer and more efficient.

Firewood Seasoning Techniques and Safety Considerations

Seasoning firewood is the process of drying wood to reduce its moisture content. Seasoned firewood burns more efficiently and produces more heat.

Stacking: Stack firewood in a single row, off the ground, to allow air to circulate. Cover the top of the stack to protect it from rain and snow.
Location: Choose a sunny, well-ventilated location for your firewood stack.
Time: Allow firewood to season for at least six months, and preferably a year. Hardwoods take longer to season than softwoods.
Moisture Meter: Use a moisture meter to check the moisture content of your firewood. Aim for a moisture content of 20% or less.
Safety Considerations: Wear gloves and eye protection when handling firewood. Be aware of the potential for insect infestations.
Data Point: A study by a firewood retailer found that customers were willing to pay 20% more for seasoned firewood than for green firewood.
My Method: I stack my firewood in a single row on pallets to keep it off the ground. I cover the top of the stack with a tarp to protect it from rain. I use a moisture meter to check the moisture content before burning it.
Tip: Seasoning firewood is not only about drying it; it’s also about allowing the wood to undergo chemical changes that make it burn more efficiently.

Seasoning firewood is an essential step in preparing firewood for burning.

Project Planning and Execution

Before starting any wood processing or firewood preparation project, it’s important to plan the project carefully.

Assessment: Assess the scope of the project. How much wood needs to be processed? What tools and equipment are needed?
Safety: Identify potential hazards and take steps to mitigate them. Wear appropriate safety gear, such as gloves, eye protection, and hearing protection.
Timeline: Create a timeline for the project. How long will it take to complete?
Budget: Develop a budget for the project. How much will it cost to purchase the necessary tools and materials?
Execution: Execute the project according to the plan. Be flexible and adapt to changing conditions.
Data Point: A study by a construction management firm found that projects that were carefully planned were 30% more likely to be completed on time and within budget.
My Approach: Before starting any wood processing project, I create a detailed plan that includes a list of tasks, a timeline, a budget, and a safety checklist.
Real-World Example: I once started a firewood preparation project without properly planning it. I ran out of firewood storage space halfway through the project, and I had to scramble to find additional storage. I learned my lesson: always plan ahead.

Careful project planning and execution will help you complete your wood processing and firewood preparation projects safely and efficiently.

Conclusion: Taking the Next Steps

Rebuilding an MS390 chainsaw can seem daunting at first, but with the right knowledge, tools, and a bit of patience, it’s a very achievable task. Remember to start with a thorough diagnosis, choose quality parts, follow the rebuild process carefully, and tune the engine for optimal performance. And most importantly, practice proactive maintenance to keep your MS390 running strong for years to come.

So, what’s next? If your MS390 is showing signs of wear, don’t wait until it’s completely broken. Start the diagnosis process now. Order a rebuild kit from a reputable supplier. Clear your workspace, gather your tools, and get ready to breathe new life into your trusty chainsaw. You’ll not only save money but also gain a deeper understanding of how your chainsaw works. And that, my friends, is a valuable skill for any wood processing enthusiast. Happy cutting!

Chainsaw Rebuild Kit: Top Tips for MS390 Repair (5 Pro Secrets)