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Start ShoppingCometic C9166 Valve Stem Seal for Harley-Davidson Evolution Big Twin/Twin Cam - High-Performance Gasket
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Description
Ensure optimal engine performance and reliability with Cometic's high-quality valve stem seals. These critical components help control oil consumption and prevent oil from entering the combustion chamber, delivering smoother operation and extended engine life for your Harley-Davidson Evolution Big Twin or Twin Cam motorcycle.
Key Features
- Positive-Style Valve Stem Seal Design: Provides superior sealing action to maintain compression and reduce oil burn-off.
- Oil Control Technology: Engineered to minimize oil consumption while preventing harmful oil contamination in the combustion chamber.
- Rebuild-Ready Compatibility: Specifically designed for use during cylinder head rebuilds or head refreshes on specified Harley-Davidson engine families.
Why Choose This Product
Cometic's valve stem seals offer unmatched durability and precision engineering for powersports applications. Their advanced sealing technology helps maintain engine efficiency, reduce maintenance costs, and improve overall performance. Whether you're restoring a classic Twin Cam or upgrading your Evolution Big Twin, these seals provide reliable protection against oil leaks and combustion chamber contamination. Always verify exact specifications for part C9166 by consulting Cometic's official documentation for optimal results.
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Ensure optimal engine performance and reliability with Cometic's high-quality valve stem seals. These critical components help control oil consumption and prevent oil from entering the combustion chamber, delivering smoother operation and extended engine life for your Harley-Davidson Evolution Big Twin or Twin Cam motorcycle.
Key Features
- Positive-Style Valve Stem Seal Design: Provides superior sealing action to maintain compression and reduce oil burn-off.
- Oil Control Technology: Engineered to minimize oil consumption while preventing harmful oil contamination in the combustion chamber.
- Rebuild-Ready Compatibility: Specifically designed for use during cylinder head rebuilds or head refreshes on specified Harley-Davidson engine families.
Why Choose This Product
Cometic's valve stem seals offer unmatched durability and precision engineering for powersports applications. Their advanced sealing technology helps maintain engine efficiency, reduce maintenance costs, and improve overall performance. Whether you're restoring a classic Twin Cam or upgrading your Evolution Big Twin, these seals provide reliable protection against oil leaks and combustion chamber contamination. Always verify exact specifications for part C9166 by consulting Cometic's official documentation for optimal results.
| SKU | GM7D6E3FB2 |
|---|---|
| MPN | C9166 |
| Brand | Cometic |
| Category | Gaskets/Seals |
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Frequently Asked Questions
It is likely designed as an OEM‑style replacement for specific Evo Big Twin / Twin Cam applications, but exact coverage for C9166 must be confirmed in the Cometic catalog or with your VIN/engine code. Do not rely on generic Evo/Twin Cam labeling alone.
Most shops remove the heads for proper valve and guide inspection, but experienced technicians can change seals on‑engine using a valve spring compressor and compressed air to hold the valves. Follow a factory service manual for your exact engine.
Possibly, but high‑lift cams and taller springs may require low‑profile or specific performance seals. Check installed height and retainer‑to‑seal clearance; consult the cam/spring manufacturer and Cometic tech support with your exact part numbers.
Fresh valve stem seals, when installed correctly on valves and guides that are still in spec, often reduce oil consumption and blue smoke on startup or decel. If guides are excessively worn, seals alone may not solve the issue.
Yes. When the heads are apart, replace the full set of intake and exhaust seals so wear is even and you don’t have to reopen the engine soon after.
Installation Guide
Difficulty
Medium
Est. Time
1 hr 0 min
Tools Required
- Valve spring compressor (for Harley Evo/Twin Cam style heads)
- Small pick or seal removal tool
- Seal installation sleeve or appropriately sized deep socket
- Torque wrench (for reassembling rocker boxes / head components)
- Shop rags and engine assembly lube
Pro Tips
- Use a thin installation sleeve or a bit of tape over valve keeper grooves so the seal lip is not cut.
- Lightly oil the valve stem and seal ID with clean engine oil before install.
- Press the seal straight down until you feel a positive ‘click’ or firm stop on the guide boss.
- Work on one valve at a time and keep intake/exhaust hardware clearly separated.
- After assembly, rotate the engine by hand to ensure smooth valve movement and that retainers do not contact seals.
Common Mistakes to Avoid
- Damaging the seal lip while installing over valve stem locks/keepers
- Not fully seating the seal on the guide boss
- Mixing intake and exhaust seals if they are different part numbers or colors
- Reusing worn valve guides or valves, which can cause premature seal failure
- Contaminating new seals with grit or solvent during install
Comparison vs Stock/OEM
Cometic valve stem seals are generally designed as high‑quality OEM‑style or performance replacements. They typically aim for improved oil control and durability compared with many factory seals.
Improvements
- Better oil control when guides and valves are within spec
- High‑quality elastomer materials designed for modern oils and temperatures (exact material for C9166 not verified)
- Positive‑type design for secure fit on valve guide boss
Considerations
- Must be correctly matched to guide diameter and installed height.
- Improper installation can negate any benefits and cause leaks or premature failure.
Durability & Warranty
Expected Lifespan
Typically expected to last for many tens of thousands of miles when used with in‑spec guides/valves and proper lubrication, comparable to or better than OEM seals.
Warranty
Specific warranty terms for Cometic C9166 were not found; many gasket/seal manufacturers offer limited warranties against manufacturing defects only. Confirm with Cometic or your distributor.
Maintenance
No routine maintenance of the seals themselves; monitor oil consumption and exhaust smoke. During future top‑end service, replace seals again rather than reusing.
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