Oil Seals Chainsaw STIHL MS 170 – Complete Guide (2025)
The STIHL MS 170 is one of the most popular entry-level chainsaws in the world, known for its lightweight design, reliability, and affordable price. However, like all small engines, it relies on precision components to maintain air-tight and fuel-tight operation—one of the most critical being the oil seals. In this comprehensive guide, we’ll explore everything you need to know about oil seals chainsaw STIHL MS 170 — including their function, symptoms of wear, replacement procedures, part numbers, and maintenance tips.
What Are Oil Seals on a STIHL Chainsaw?
The oil seals on a chainsaw are small but vital components that prevent air leaks and ensure the crankcase remains sealed. In the STIHL MS 170, the oil seals are located at both ends of the crankshaft — one near the flywheel and the other near the clutch side.
Their main purpose is to:
- Prevent air leaks into the crankcase (which can cause lean running or poor idling).
- Stop fuel/oil mixture from leaking out of the engine.
- Maintain proper crankcase pressure, which is essential for correct two-stroke operation.
Without properly functioning oil seals, your STIHL MS 170 will lose power, idle inconsistently, or fail to start altogether. STIHL 500i Chainsaw: The Ultimate Power Tool for Professionals
Symptoms of Bad Oil Seals on a STIHL MS 170
If your chainsaw isn’t performing as it should, the oil seals chainsaw STIHL MS 170 may be the culprit. Here are the most common symptoms of worn or leaking seals:
1. Engine Racing or High Idle
A leaking oil seal allows extra air into the crankcase, leaning out the fuel mixture and causing the engine to race uncontrollably or idle too high.
2. Hard Starting
Leaking crank seals can make the air-fuel ratio unstable, leading to difficult starting—especially when the engine is warm.
3. Loss of Power
If your STIHL MS 170 lacks power or dies when cutting, it may be because air leaks are affecting combustion efficiency.
4. Chain Oiler Not Working Properly
In some cases, oil may leak around the clutch side, indicating the seal near the oil pump is damaged.
5. Visible Oil Leakage
Oil dripping from behind the flywheel or clutch area is a strong indicator of failed seals.
6. Engine Won’t Idle or Stalls
If the saw runs but won’t hold idle, air leaks from the oil seals can cause unbalanced internal pressure.
Causes of Oil Seal Failure
Over time, STIHL MS 170 oil seals can wear out due to several reasons:
- Heat and friction: Continuous engine heat causes rubber seals to harden or shrink.
- Old fuel or poor lubrication: Low-quality or stale fuel can degrade rubber components.
- Extended storage: Drying out during long storage periods without fuel or oil.
- Improper installation: A misaligned crankshaft or damaged bearing can cause premature seal wear.
- Dirt and debris: Fine dust and sawdust entering the crank area can damage the seals.
Checking Oil Seals on a STIHL MS 170
If you suspect your oil seals are bad, there are a few simple ways to test them before disassembly:
1. Visual Inspection
Remove the clutch cover and flywheel side housing. Check for oil residue or wet spots near the crankshaft exits. If you see oil buildup, it’s a likely sign the seal is leaking.
2. Pressure or Vacuum Test
A professional way to confirm seal condition is to perform a crankcase pressure/vacuum test. This requires sealing the intake and exhaust ports, then using a pressure tester to see if the crankcase holds pressure.
- If pressure drops quickly, the oil seals chainsaw STIHL MS 170 are leaking.
STIHL MS 170 Oil Seals Part Numbers
STIHL provides specific part numbers for the oil seals used on the MS 170 model:
- Flywheel Side Oil Seal: Part No. 9639 003 1740
- Clutch Side Oil Seal: Part No. 9639 003 1741
- Crankshaft Bearing Set (if needed): Part No. 9503 003 0316
It’s highly recommended to use OEM STIHL parts rather than cheap aftermarket alternatives, as incorrect seals can cause improper sealing or faster wear.
Tools and Materials Needed for Replacement
To replace oil seals on STIHL MS 170, you’ll need:
- Flywheel puller
- Clutch removal tool (STIHL #5910 890 4500 or equivalent)
- Small screwdriver or seal puller
- Heat gun or hairdryer (for loosening seals)
- Socket wrench set
- New crankcase oil seals (OEM recommended)
- Two-stroke oil or grease (for lubricating new seals)
- Torque wrench (for proper reassembly)
How to Replace Oil Seals on STIHL MS 170 (Step-by-Step)
Replacing oil seals on STIHL MS 170 requires some mechanical skill but can be done at home with care. Below is a step-by-step guide:
Step 1: Preparation
- Disconnect the spark plug and drain fuel and bar oil.
- Remove the bar and chain for safety.
Step 2: Remove the Clutch and Flywheel
- Use a piston stop tool or rope in the spark plug hole to prevent crankshaft rotation.
- Unscrew and remove the clutch (note that STIHL clutches are left-hand threaded).
- Remove the flywheel using a puller.
Step 3: Access the Oil Seals
- Once both sides are exposed, you’ll see the oil seals pressed into the crankcase.
- Check for any oil residue or dirt buildup.
Step 4: Remove Old Seals
- Carefully pry out the old seals using a seal puller or small screwdriver.
- Be gentle—avoid scratching the crankshaft or crankcase surface.
Step 5: Install New Seals
- Lightly coat the new STIHL MS 170 oil seals with two-stroke oil or grease.
- Press them evenly into place using a socket that matches the outer diameter of the seal.
- Ensure they sit flush and are not twisted.
Step 6: Reassemble the Chainsaw
- Reinstall the flywheel and clutch, torquing them to factory specifications.
- Reattach the bar, chain, and covers.
- Add fresh fuel and oil, and test-run the saw.
If the saw idles smoothly and no longer races or leaks oil, the repair was successful.
Maintenance Tips to Prolong Oil Seal Life
Preventive care can help extend the lifespan of your STIHL MS 170 oil seals:
- Use quality two-stroke oil – Preferably STIHL HP Ultra or synthetic oil.
- Avoid old fuel – Replace fuel every 30 days to prevent gumming.
- Run your saw regularly – Prevents seals from drying out.
- Keep the air filter clean – Reduces debris entry into the crank area.
- Store in a dry, cool place – Avoid prolonged exposure to heat or sunlight.
When to Replace the Oil Seals
Generally, oil seals on STIHL MS 170 last for many years if maintained properly. However, you should consider replacement when:
- The saw is over 5–7 years old and showing idle issues.
- You’re already doing a major rebuild (piston, crank, bearings).
- You see visible oil leaks around crankshaft ends.
- Pressure test indicates loss of vacuum or pressure.
DIY vs. Professional Replacement
If you’re mechanically inclined, replacing oil seals chainsaw STIHL MS 170 yourself can save labor costs (which average $75–$120 at a STIHL service center).
However, if you lack tools or experience, a professional technician ensures:
- Proper crankcase sealing.
- Correct torque settings.
- Pressure testing after reassembly.
This guarantees the longevity and reliability of your saw.
OEM vs. Aftermarket Oil Seals
While aftermarket seals are cheaper, they often don’t meet STIHL’s specifications for temperature, pressure, or material quality.
OEM STIHL seals are made of high-grade rubber compounds and fit perfectly without modification. Investing in genuine parts prevents future air leaks and costly engine damage.
Common Mistakes During Replacement
- Damaging the crankshaft surface while prying seals.
- Installing seals backwards (spring side must face inward).
- Failing to lubricate the seal lip before installation.
- Forgetting to pressure test after assembly.
- Using cheap or incompatible seals.
Each of these errors can lead to premature seal failure or engine performance issues.
Where to Buy STIHL MS 170 Oil Seals
You can purchase genuine STIHL MS 170 oil seals from:
- Authorized STIHL dealers (recommended source)
- STIHL USA website
- Reputable online parts stores like Chainsawr, Jack’s Small Engines, or HL Supply
- Local hardware or small engine repair shops
Always confirm the part numbers to ensure compatibility with your specific MS 170 model year.
Conclusion: Keep Your STIHL MS 170 Running Strong
The oil seals chainsaw STIHL MS 170 may be small, but they play a critical role in your saw’s performance and reliability. Neglecting worn seals can lead to air leaks, poor idling, or even catastrophic engine failure. By recognizing symptoms early, using genuine parts, and following proper installation steps, you can restore your saw to factory condition and extend its working life for years.
Regular maintenance, proper fuel, and careful storage will help you avoid oil seal problems altogether. Whether you’re a weekend woodcutter or a professional arborist, keeping your STIHL MS 170 in top shape starts with maintaining these essential seals.
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