Stihl MS 251 Bar Guide (5 Pro Tips for Perfect Chain Tension)

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Understanding the Importance of Chain Tension

Chain tension is the Goldilocks of chainsaw maintenance – it can’t be too tight, and it can’t be too loose; it has to be just right. Too tight, and you’ll experience increased friction, leading to accelerated wear on the bar, chain, and sprocket. You might even burn out the clutch or damage the engine. Too loose, and the chain can derail, causing potential injury and damaging the saw. A loose chain also tends to “slap” against the bar, leading to uneven wear and reduced cutting efficiency.

Think of it this way: a properly tensioned chain is like a well-tuned musical instrument. Each component works in harmony to produce the desired result – in this case, efficient and safe cutting. Neglecting chain tension is like playing a guitar with loose strings – it just won’t sound right, and you’ll likely break something in the process.

Tip #1: The “Pull-and-Snap” Test: Your Quick Reference Guide

The most common method for checking chain tension is the “pull-and-snap” test. This is a quick visual and tactile assessment that experienced users can perform in seconds.

How to Perform the Pull-and-Snap Test:

1. Safety First: Always wear gloves when handling a chainsaw chain. The teeth are sharp, and even a seemingly dull chain can inflict a nasty cut.
2. Position the Saw: Place the chainsaw on a flat surface, with the bar facing up.
3. Pull the Chain: Using your gloved hand, gently pull the chain away from the bar at the midpoint of the bar’s underside.
4. Observe the Gap: The chain should pull away from the bar enough to expose approximately one or two drive links (the small, tooth-like projections that fit into the bar’s groove).
5. Snap Test: Release the chain. It should snap back against the bar with a distinct “ting” or “snap” sound.

Interpreting the Results:

• Too Loose: If you can pull the chain further away from the bar, exposing more than two drive links, the chain is too loose. You’ll also notice excessive sagging.
• Too Tight: If you can barely pull the chain away from the bar, or if it feels stiff and doesn’t snap back easily, the chain is too tight.
• Just Right: The chain pulls away slightly, exposing one or two drive links, and snaps back against the bar with a crisp sound.

Personal Story: I remember one time, I was helping a friend cut firewood, and he kept complaining about the saw bogging down. After a quick inspection, I realized his chain was so loose it was practically falling off the bar. A simple tension adjustment made a world of difference, and he was back to cutting efficiently in no time.

Takeaway: The pull-and-snap test is a quick and easy way to check chain tension in the field. Make it a habit to perform this test before each use and periodically throughout the day.

Tip #2: Mastering the Tensioning Process: A Step-by-Step Guide

The Stihl MS 251 features a side-mounted chain tensioner, which makes the adjustment process relatively straightforward. However, it’s essential to follow the correct procedure to ensure accurate and safe tensioning.

Tools You’ll Need:

• Combination Wrench/Scrench: This tool, often included with the chainsaw, is used to loosen the bar nuts and adjust the chain tensioning screw.
• Gloves: For safety.

Step-by-Step Tensioning Guide:

1. Safety First: Engage the chain brake. This prevents the chain from moving during the adjustment process.
2. Loosen the Bar Nuts: Use the scrench to slightly loosen the bar nuts on the side of the chainsaw. Don’t remove the nuts completely; just loosen them enough so the bar can move. The exact amount will vary, but generally, a quarter to half turn is sufficient.
3. Locate the Tensioning Screw: The tensioning screw is typically located on the side of the chainsaw, near the bar. On the MS 251, it’s usually a small, recessed screw that you can access with the scrench.
4. Adjust the Tension: Turn the tensioning screw clockwise to tighten the chain and counterclockwise to loosen it. Make small adjustments and check the tension after each adjustment using the pull-and-snap test.
5. Tighten the Bar Nuts: Once you’ve achieved the correct chain tension, tighten the bar nuts securely. Ensure the bar is properly seated against the saw body before tightening. I usually tighten them in an alternating pattern (like tightening lug nuts on a car wheel) to ensure even pressure.
6. Release the Chain Brake: Disengage the chain brake and manually rotate the chain around the bar to ensure it moves freely. Re-check the tension after rotating the chain.

Important Considerations:

• New Chains: New chains tend to stretch during the first few uses. Check the tension frequently during the break-in period and readjust as needed.
• Warm Chains: The chain will expand as it heats up during use. It’s best to adjust the tension when the chain is at operating temperature. If you adjust a cold chain, it may become too tight when it warms up.
• Bar Alignment: Ensure the bar is properly aligned with the saw body before tightening the bar nuts. Misalignment can lead to uneven chain wear and poor cutting performance.

Takeaway: Mastering the chain tensioning process is crucial for safe and efficient chainsaw operation. Practice the steps outlined above until you can confidently adjust the chain tension on your Stihl MS 251.

Tip #3: Understanding Chain Stretch: Why Frequent Adjustments Are Necessary

Chain stretch is a natural phenomenon that occurs as the chain wears and the metal elongates under stress. This is especially pronounced with new chains, as the manufacturing tolerances settle in. Understanding chain stretch is crucial for maintaining proper chain tension and preventing premature wear.

Factors Contributing to Chain Stretch:

• Chain Quality: Higher-quality chains tend to stretch less than cheaper chains. Look for chains made from durable alloys with hardened components.
• Cutting Conditions: Cutting dirty or abrasive wood can accelerate chain wear and stretch.
• Lubrication: Insufficient lubrication increases friction, leading to accelerated wear and stretch.
• Chain Tension: Operating with a chain that is consistently too tight or too loose can accelerate stretch.

How to Manage Chain Stretch:

• Frequent Checks: Check the chain tension frequently, especially when using a new chain or cutting in challenging conditions.
• Regular Adjustments: Adjust the chain tension as needed to compensate for stretch.
• Proper Lubrication: Ensure the chain is adequately lubricated at all times. Use a high-quality bar and chain oil. I prefer to use a biodegradable oil when possible, to minimize environmental impact.
• Chain Rotation: If you use multiple chains, rotate them regularly to distribute wear evenly.
• Chain Sharpening: A sharp chain cuts more efficiently, reducing the strain on the chain and minimizing stretch.
• Bar Maintenance: Keep the bar clean and free of burrs. A damaged bar can contribute to chain stretch.

Data and Insights: I’ve tracked chain stretch on several of my chains over the years. On average, a new chain will stretch significantly during the first 5-10 hours of use, requiring frequent adjustments. After that, the rate of stretch typically slows down. However, even well-maintained chains will eventually reach the end of their useful life due to excessive stretch.

Case Study: I once worked on a logging project where we were cutting extremely hard and abrasive wood. The chains were stretching so rapidly that we had to adjust them every 15-20 minutes. We eventually switched to a higher-quality chain specifically designed for abrasive conditions, which significantly reduced the rate of stretch.

Takeaway: Chain stretch is an unavoidable part of chainsaw operation. By understanding the factors that contribute to chain stretch and implementing the management strategies outlined above, you can minimize its impact and extend the life of your chains.

Tip #4: The Importance of Bar Maintenance: A Foundation for Perfect Chain Tension

The chainsaw bar is the foundation upon which the chain operates. A worn or damaged bar can compromise chain tension, leading to poor cutting performance and accelerated wear on the chain and saw. Regular bar maintenance is essential for achieving and maintaining perfect chain tension.

Key Aspects of Bar Maintenance:

• Cleaning: Keep the bar clean and free of sawdust, pitch, and debris. Use a wire brush or a scraper to remove accumulated build-up.
• Deburring: Check the bar rails (the edges of the bar that guide the chain) for burrs or damage. Use a bar dressing tool or a flat file to remove any burrs or sharp edges.
• Rail Trueness: Ensure the bar rails are straight and parallel. Use a bar rail gauge to check for wear or distortion. If the rails are significantly worn or uneven, the bar should be replaced.
• Lubrication Hole Cleaning: The bar has small lubrication holes that allow oil to reach the chain. These holes can become clogged with debris. Use a small wire or a needle to clear any blockages.
• Bar Flipping: Regularly flip the bar over to distribute wear evenly on both sides. This can significantly extend the life of the bar.
• Sprocket Inspection: Check the sprocket (the toothed wheel that drives the chain) for wear or damage. A worn sprocket can cause uneven chain wear and poor cutting performance. Replace the sprocket as needed.

Tools for Bar Maintenance:

• Wire Brush: For cleaning the bar.
• Bar Dressing Tool: For removing burrs and dressing the bar rails.
• Bar Rail Gauge: For checking the trueness of the bar rails.
• Small Wire or Needle: For cleaning lubrication holes.
• Scrench: For removing and installing the bar.

Personal Experience: I once neglected to clean the lubrication holes on my bar, and the chain started to overheat and stretch excessively. I quickly realized the problem, cleaned the holes, and the issue was resolved. This experience taught me the importance of regular bar maintenance.

Actionable Metrics:

• Bar Cleaning Frequency: Clean the bar after each use or at least once a day when using the chainsaw frequently.
• Deburring Frequency: Deburr the bar rails whenever you notice burrs or sharp edges.
• Bar Flipping Frequency: Flip the bar over every time you sharpen the chain.
• Sprocket Replacement: Replace the sprocket after every two or three chain replacements, or sooner if it shows signs of wear.

Takeaway: Bar maintenance is an integral part of chainsaw maintenance. By following the guidelines outlined above, you can ensure your bar is in optimal condition, which will contribute to perfect chain tension, improved cutting performance, and extended equipment life.

Why a Hot Saw Adjustment Matters:

As the chain heats up during use, it expands. If you adjust the chain tension when the saw is cold, it may become too tight when it warms up, leading to increased friction and potential damage. The hot saw adjustment compensates for this expansion, ensuring the chain is properly tensioned under operating conditions.

How to Perform a Hot Saw Adjustment:

1. Warm Up the Saw: Run the chainsaw under normal cutting conditions for several minutes to allow the chain to heat up and expand.
2. Engage the Chain Brake: Stop the saw and engage the chain brake.
3. Loosen the Bar Nuts: Slightly loosen the bar nuts, as described in Tip #2.
4. Adjust the Tension: Adjust the chain tension using the tensioning screw. The goal is to achieve a slightly looser tension than you would normally set when the saw is cold. The chain should still pass the pull-and-snap test, but it should have a bit more slack.
5. Tighten the Bar Nuts: Tighten the bar nuts securely.
6. Release the Chain Brake: Disengage the chain brake and manually rotate the chain around the bar to ensure it moves freely.
7. Re-Check the Tension: After the saw cools down, re-check the chain tension. It should be slightly tighter than it was when hot, but still within the acceptable range.

Important Considerations:

• Experience Required: The hot saw adjustment requires experience and a feel for how the chain behaves under different conditions. It’s best to start with small adjustments and gradually refine your technique.
• Safety Precautions: Be extremely careful when working with a hot chainsaw. Wear appropriate safety gear, including gloves and eye protection.
• Cutting Conditions: The hot saw adjustment is most beneficial when cutting large-diameter wood or working under heavy loads. It may not be necessary for light-duty cutting.
• Chain Type: Different chain types may expand at different rates. Adjust your technique accordingly.

Real-World Example: I often use the hot saw adjustment when felling large trees. The increased load on the chain during these operations causes it to heat up and expand significantly. By performing a hot saw adjustment, I can ensure the chain is properly tensioned throughout the felling process, minimizing friction and maximizing cutting efficiency.

Takeaway: The hot saw adjustment is an advanced technique that can help you fine-tune chain tension for peak performance. However, it requires experience and a thorough understanding of chainsaw operation. Use this technique with caution and always prioritize safety.

Bonus Tip: The Sound of a Properly Tensioned Chain

Beyond the visual and tactile tests, an experienced chainsaw user can often tell if the chain tension is correct simply by listening to the sound of the saw. A properly tensioned chain will produce a smooth, consistent whirring sound when running. A loose chain will often make a slapping or rattling noise, while a chain that is too tight may produce a high-pitched whining sound. Pay attention to the sounds your chainsaw makes, and you’ll develop a better sense of when the chain tension needs adjustment.

Final Thoughts

Maintaining perfect chain tension on your Stihl MS 251 is not just about improving cutting performance; it’s about ensuring your safety and extending the life of your equipment. By mastering the techniques and tips outlined in this guide, you’ll be well-equipped to keep your chainsaw running smoothly and efficiently for years to come. Remember to always prioritize safety, and don’t hesitate to consult with a qualified chainsaw technician if you have any questions or concerns. Happy cutting!

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