Stihl 420 Chainsaw Restoration Tips (5 Expert Rebuild Tricks)

It was a cold November afternoon in the Oregon Coast Range. The smell of Douglas fir hung heavy in the air, mixed with the unmistakable scent of two-stroke engine exhaust. My hands, raw and calloused, were stained with bar oil and sawdust. I was 17, wrestling with a stubborn Stihl 041 Farm Boss that had seen better days. Back then, “restoration” meant cleaning the spark plug and hoping for the best. Little did I know, that beaten-up saw would ignite a lifelong passion for chainsaws and wood processing. Years later, I’ve rebuilt dozens of saws, from vintage relics to modern marvels. One model that consistently captures the hearts of enthusiasts and demands respect is the Stihl MS 420. It’s a powerful, reliable workhorse, and restoring one is a deeply satisfying experience. Today, I’m going to share five expert rebuild tricks that I’ve learned along the way, tricks that will help you breathe new life into your own Stihl MS 420.

Stihl MS 420 Chainsaw Restoration: 5 Expert Rebuild Tricks

The Stihl MS 420 is a mid-range professional saw, known for its robust construction and impressive power-to-weight ratio. It’s a popular choice for homeowners, farmers, and even some professional loggers tackling smaller jobs. However, like any machine subjected to harsh conditions and demanding workloads, it eventually needs some TLC. Restoration isn’t just about making it look pretty; it’s about ensuring peak performance, longevity, and, most importantly, safety.

Before we dive into the specific tricks, let’s clarify some key terms:

• Green Wood: Freshly cut wood with a high moisture content (often above 30%). It’s heavier and harder to split than seasoned wood.
• Seasoned Wood: Wood that has been dried to a lower moisture content (typically 15-20%). It’s lighter, easier to split, and burns more efficiently.
• Top End: The upper portion of the engine, including the cylinder, piston, and rings.
• Bottom End: The lower portion of the engine, including the crankshaft, connecting rod, and bearings.
• Carburetor: The device that mixes air and fuel in the correct proportions for combustion.
• Impulse Line: A small hose that transmits pressure pulses from the crankcase to the carburetor, helping to draw fuel.

Now, let’s get our hands dirty!

1. The Unsung Hero: Crankshaft Seal Replacement

This is, in my opinion, the most overlooked aspect of chainsaw restoration. Everyone focuses on the piston, cylinder, and carburetor, but worn crankshaft seals can silently rob your saw of power and lead to engine damage.

Why are crankshaft seals so important? They prevent air leaks into the crankcase. Air leaks disrupt the precise air/fuel mixture, leading to a lean condition. A lean condition causes the engine to run hot, potentially scoring the piston and cylinder walls and ultimately causing catastrophic failure.

The telltale signs of worn crankshaft seals:

• Difficult starting: The engine might require excessive pulling or refuse to start at all.
• Erratic idling: The engine might idle too high or stall frequently.
• Poor performance: The saw might lack power, especially under load.
• Visible wear: Cracks or brittleness in the seals themselves (often difficult to see without disassembly).

The process (Stihl MS 420 specific):

1. Disassembly: This requires removing the clutch (on the clutch side) and the flywheel (on the ignition side). You’ll need a piston stop tool to prevent the crankshaft from turning while you loosen the clutch and flywheel nuts. Remember, the clutch is typically left-hand threaded. A specialized clutch removal tool is highly recommended to avoid damaging the clutch assembly. I’ve used a universal clutch removal tool from Oregon (part number 27920) with great success.
2. Seal Removal: Carefully pry out the old seals. I prefer using a seal removal tool, but a small screwdriver can work in a pinch. Be extremely careful not to scratch the crankshaft or the surrounding metal. I once used a pick, slipped, and scored the crankshaft housing, leading to an even bigger repair. Learn from my mistake!
3. Cleaning: Thoroughly clean the seal surfaces with a solvent like carburetor cleaner. Ensure there is no residue or debris that could prevent the new seals from seating properly.
4. Installation: Lightly lubricate the new seals with two-stroke oil. Use a seal installation tool or a socket that matches the outside diameter of the seal to gently tap it into place. Ensure the seal is seated flush and square. I prefer using Stihl OEM seals for their superior quality and durability.
5. Reassembly: Reinstall the clutch and flywheel, tightening the nuts to the manufacturer’s specified torque (refer to the Stihl MS 420 service manual).

Tools needed:

• Piston stop tool
• Clutch removal tool
• Seal removal tool (optional)
• Seal installation tool or socket
• Torque wrench
• Two-stroke oil
• Carburetor cleaner
• Gloves

Cost: Stihl OEM crankshaft seals typically cost around $10-15 each. The tools (piston stop, clutch removal, seal tools) can range from $20 to $50 depending on the quality and brand.

Strategic Advantage: Replacing the crankshaft seals is a relatively inexpensive repair that can significantly extend the life of your Stihl MS 420 and improve its performance. It’s a preventative measure that can save you from more costly repairs down the road.

2. Carburetor Rebuild: Beyond the Basics

A clean carburetor is essential for proper engine operation. While most people simply clean the carburetor, a complete rebuild is often necessary for older saws.

Why rebuild the carburetor? Over time, the rubber and plastic components inside the carburetor can harden, crack, or swell, leading to fuel leaks, poor fuel metering, and ultimately, engine problems.

The process (Walbro or Zama carburetor specific to the MS 420):

1. Disassembly: Carefully disassemble the carburetor, noting the location of each component. Refer to a carburetor diagram for your specific model (Walbro or Zama). Pay close attention to the position of the metering lever.
2. Cleaning: Thoroughly clean all carburetor components with carburetor cleaner. Use compressed air to blow out all passages and orifices. I use a set of carburetor cleaning picks to ensure that no debris is left behind.
3. Inspection: Inspect the metering lever, needle valve, and seat for wear or damage. Replace these components if necessary. Check the condition of the diaphragms and gaskets. They should be pliable and free from cracks or tears.
4. Rebuild Kit: Use a high-quality carburetor rebuild kit that includes all the necessary diaphragms, gaskets, and needle valve. Stihl OEM kits are recommended, but aftermarket kits from reputable brands like Walbro or Zama are also acceptable.
5. Reassembly: Reassemble the carburetor according to the diagram. Pay close attention to the metering lever height. The correct height is crucial for proper fuel metering. The service manual will specify the correct height (usually measured in millimeters). I use a small ruler and a magnifying glass to ensure accurate adjustment.
6. Adjustment: After reassembly, you’ll need to adjust the high (H) and low (L) speed needles. Start with the factory settings (usually 1 turn out from fully closed) and fine-tune as needed. Use a tachometer to ensure the engine is running at the correct RPM.

Tools needed:

• Carburetor cleaner
• Compressed air
• Carburetor cleaning picks
• Small screwdrivers
• Needle-nose pliers
• Carburetor rebuild kit
• Tachometer (optional)

Cost: Carburetor rebuild kits typically cost between $15 and $30. A tachometer can range from $30 to $100.

Strategic Advantage: A properly rebuilt carburetor will ensure optimal fuel metering, resulting in improved engine performance, fuel efficiency, and reduced emissions. It will also make your saw easier to start and more reliable.

Personal Experience: I once rebuilt a carburetor on a Stihl MS 420 that had been sitting unused for years. The original diaphragms were so brittle that they crumbled to the touch. After the rebuild, the saw started on the second pull and ran like new. It was a testament to the importance of a properly functioning carburetor.

3. The Power of Porting: Unleashing Hidden Potential

This is where things get a little more advanced. Porting involves modifying the intake and exhaust ports of the cylinder to improve airflow and increase engine power.

What is porting? Porting essentially enlarges and reshapes the ports to allow more air and fuel to enter the cylinder and more exhaust gases to exit. This results in a more efficient combustion process and increased horsepower.

Is porting right for you? Porting requires specialized tools, knowledge, and experience. It’s not for beginners. If you’re not comfortable working on engines, it’s best to leave this to a professional. However, if you’re an experienced mechanic looking to take your Stihl MS 420 to the next level, porting can be a rewarding endeavor.

The process (requires extreme precision):

1. Research: Thoroughly research porting techniques for the Stihl MS 420. There are many online resources and forums dedicated to chainsaw porting. Study the port maps and understand the principles of airflow dynamics.
2. Tools: You’ll need a rotary tool (like a Dremel) with various grinding stones and sanding drums. A die grinder with carbide cutters is also helpful for more aggressive porting. Eye protection is absolutely crucial. I also recommend wearing a respirator to avoid inhaling metal dust.
3. Port Mapping: Use a port map as a guide. Port maps show the ideal shape and size of the ports. You can find port maps online or create your own by carefully measuring the existing ports.
4. Grinding: Carefully grind away material from the ports, following the port map. Work slowly and methodically, checking your progress frequently. It’s better to remove too little material than too much.
5. Polishing: After grinding, polish the ports to a smooth finish. This will further improve airflow.
6. Testing: After porting, reassemble the engine and test its performance. You may need to adjust the carburetor to compensate for the increased airflow.

Tools needed:

• Rotary tool (Dremel) with grinding stones and sanding drums
• Die grinder with carbide cutters (optional)
• Port maps
• Calipers
• Eye protection
• Respirator

Cost: The cost of porting tools can range from $50 to $500, depending on the quality and type of tools.

Strategic Advantage: Porting can significantly increase the power and performance of your Stihl MS 420. It’s a great way to get more out of your saw, especially if you’re using it for demanding tasks like felling large trees or milling lumber.

Caution: Porting can also damage your engine if done incorrectly. It’s important to proceed with caution and to have a thorough understanding of the process before you begin.

4. Bar and Chain Optimization: The Cutting Edge

The bar and chain are the business end of your chainsaw. Optimizing them is crucial for efficient cutting and safety.

Why is bar and chain optimization important? A dull or improperly maintained chain can reduce cutting speed, increase fuel consumption, and put undue stress on the engine. A worn bar can cause the chain to derail and increase the risk of kickback.

The process:

1. Chain Sharpening: Sharpen your chain regularly. Use a chainsaw file and a filing guide to maintain the correct cutting angles. I prefer using a Stihl 2-in-1 filing tool, which sharpens the cutters and lowers the depth gauges simultaneously.
2. Depth Gauge Adjustment: Check the depth gauges (the small protrusions in front of the cutters) and file them down if necessary. The correct depth gauge setting is crucial for efficient cutting. The service manual will specify the correct depth gauge setting for your chain type.
3. Bar Maintenance: Clean the bar regularly and remove any burrs or damage. Check the bar rails for wear and file them down if necessary. Lubricate the bar regularly with chainsaw bar oil. I prefer using a high-quality bar oil that contains tackifiers to prevent it from being thrown off the bar.
4. Chain Selection: Choose the right chain for the job. Different chain types are designed for different applications. For example, a full chisel chain is ideal for cutting clean wood, while a semi-chisel chain is more durable and better suited for cutting dirty or frozen wood.
5. Bar Selection: Choose the correct bar length for your saw and the type of wood you’re cutting. A longer bar is useful for felling large trees, while a shorter bar is more maneuverable for limbing.

Tools needed:

• Chainsaw file
• Filing guide
• Depth gauge tool
• Bar dressing tool
• Chainsaw bar oil

Cost: Chainsaw files and filing guides typically cost between $10 and $30. Bar dressing tools cost around $20. Chainsaw bar oil costs around $10 per gallon.

Strategic Advantage: A sharp, well-maintained chain and bar will improve cutting speed, reduce fuel consumption, and extend the life of your chainsaw. It will also make your saw safer to operate.

Case Study: I once worked on a logging crew that was felling large Douglas fir trees. The crew was using dull chains and neglecting bar maintenance. As a result, they were spending more time sharpening chains than cutting wood. I implemented a chain sharpening program and trained the crew on proper bar maintenance techniques. Within a week, the crew’s productivity increased by 20%. This demonstrates the significant impact that bar and chain optimization can have on efficiency.

5. Ignition System Revival: Sparking New Life

A strong spark is essential for reliable engine starting and performance. The ignition system consists of the spark plug, ignition coil, and flywheel.

Why is ignition system maintenance important? A weak spark can cause difficult starting, poor performance, and even engine misfires.

The process:

1. Spark Plug Inspection: Inspect the spark plug for wear, damage, or fouling. Clean or replace the spark plug as needed. The spark plug should have a light tan color. A black, oily spark plug indicates a rich condition, while a white spark plug indicates a lean condition.
2. Spark Plug Gap: Check the spark plug gap and adjust it to the manufacturer’s specifications. The correct gap is crucial for a strong spark. The Stihl MS 420 typically uses a spark plug with a gap of 0.020 inches (0.5 mm).
3. Ignition Coil Inspection: Inspect the ignition coil for cracks, damage, or corrosion. Use a multimeter to test the coil’s resistance. If the coil is faulty, replace it.
4. Flywheel Inspection: Inspect the flywheel for damage or corrosion. Clean the flywheel and the ignition coil magnets with a wire brush.
5. Air Gap Adjustment: Check the air gap between the flywheel and the ignition coil. The correct air gap is crucial for proper ignition timing. The Stihl MS 420 typically has an air gap of 0.012 inches (0.3 mm). Use a feeler gauge to adjust the air gap.

Tools needed:

• Spark plug wrench
• Spark plug gap tool
• Multimeter
• Feeler gauge
• Wire brush

Cost: Spark plugs typically cost between $5 and $10. Ignition coils can range from $20 to $50.

Strategic Advantage: A properly functioning ignition system will ensure reliable starting, smooth engine operation, and optimal performance.

Personal Experience: I once had a Stihl MS 420 that was difficult to start. I replaced the spark plug, but the problem persisted. I then tested the ignition coil with a multimeter and discovered that it was faulty. After replacing the coil, the saw started on the first pull and ran perfectly. This experience highlighted the importance of diagnosing ignition system problems correctly.

Final Thoughts and Next Steps

Restoring a Stihl MS 420 chainsaw is a rewarding experience that combines mechanical skill with a deep appreciation for these powerful machines. By focusing on these five expert rebuild tricks – crankshaft seals, carburetor rebuild, porting (with caution), bar and chain optimization, and ignition system revival – you can breathe new life into your saw and enjoy years of reliable service.

Next Steps:

1. Assess your saw: Carefully inspect your Stihl MS 420 to identify any areas that need attention.
2. Gather your tools and supplies: Make sure you have all the necessary tools and parts before you begin the restoration process.
3. Follow the steps outlined in this guide: Work methodically and carefully, paying close attention to detail.
4. Consult the service manual: The Stihl MS 420 service manual is an invaluable resource for detailed information and specifications.
5. Seek expert advice: If you’re unsure about any aspect of the restoration process, don’t hesitate to seek advice from a qualified mechanic or experienced chainsaw enthusiast.

Remember, safety is paramount. Always wear appropriate safety gear, including eye protection, gloves, and hearing protection. Work in a well-ventilated area and follow all safety precautions outlined in the Stihl MS 420 owner’s manual.

With a little patience, skill, and the right knowledge, you can transform a tired old chainsaw into a powerful and reliable workhorse. Happy sawing!

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