Powerhead Base Gasket – Fits Mercury/Mariner 4–9.8 HP (Replaces 27-89937)
Powerhead Base Gasket – Fits Mercury/Mariner 4–9.8 HP (Replaces 27-89937) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
You may return most new, unopened items within 7 days of delivery for a full refund. We'll also pay the return shipping costs if the return is a result of our error (you received an incorrect or defective item, etc.).
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Powerhead Base Gasket – Fits Mercury/Mariner 4–9.8 HP (Replaces 27-...
Product Details
This Powerhead Base Gasket provides a precision seal for various Mercury and Mariner 4, 4.5, 7.5, and 9.8 HP single-cylinder outboard engines manufactured between 1975 and 1985. It ensures a tight fit between the powerhead and the adapter plate, preventing leaks and maintaining engine compression.
This part is a direct replacement for Sierra 18-0385, Quicksilver 27-89937, and Mercury 27-89937, offering a reliable and cost-effective alternative to OEM components.
- Mounts directly between the powerhead and adapter plate.
- Constructed from high-quality, OEM-specification materials for durability.
- Covered by a standard manufacturer parts warranty.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Sierra | 18-0385 |
| Quicksilver | 27-89937 |
| Mercury | 27-89937 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Mercury/Mariner models, it is critical to verify your engine's serial number against the ranges listed to ensure an exact match, as model years can overlap.
| Brand | Model | Rated HP | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|
| Mercury | 4 (1 CYL. PRODUCT OF USA) | 4 | 1975+ | 27-89937 | Serial #: 5595532 & Up |
| Mercury | 4 (1 CYL. PRODUCT OF USA) | 4 | 1975+ | 27-89937 | Serial #: 7143688 & Up |
| Mercury | 4 (1 CYL. PRODUCT OF USA) | 4 | 1975+ | 27-89937 | Serial #: 8068065 & Up |
| Mercury | 4 (1 CYL. PRODUCT OF USA) | 4 | 1975+ | 27-89937 | Serial #: 9258281 & Up |
| Mercury | 4.5 (1 CYL.) | 4.5 | 1975–1979 | 27-89937 | Serial #: 5595532 & Up |
| Mercury | 4.5 (1 CYL.) | 4.5 | 1975–1979 | 27-89937 | Serial #: 7147238 & Up |
| Mercury | 4.5 (1 CYL.) | 4.5 | 1975–1979 | 27-89937 | Serial #: 8068065 & Up |
| Mercury | 4.5 (1 CYL.) | 4.5 | 1975–1979 | 27-89937 | Serial #: 9258281 & Up |
| Mercury | 7.5 | 7.5 | 1976–1978 | 27-89937 | Serial #: 5206550 & Up |
| Mercury | 7.5 | 7.5 | 1976–1978 | 27-89937 | Serial #: 7125388 & Up |
| Mercury | 7.5 | 7.5 | 1976–1978 | 27-89937 | Serial #: 8057545 & Up |
| Mercury | 7.5 | 7.5 | 1976–1978 | 27-89937 | Serial #: 9217756 & Up |
| Mercury | 7.5J | 7.5 | 27-89937 | Serial #: 85A1101 and up | |
| Mercury | 9.8 | 9.8 | 1975–1978 | 27-89937 | Serial #: 5206550 & Up |
| Mercury | 9.8 | 9.8 | 1975–1978 | 27-89937 | Serial #: 7127388 & Up |
| Mercury | 9.8 | 9.8 | 1975–1978 | 27-89937 | Serial #: 8057545 & Up |
| Mercury | 9.8 | 9.8 | 1975–1978 | 27-89937 | Serial #: 9217756 & Up |
| Mercury | 39 (1 CYL.) | 3.9 | 27-89937 | Serial #: 1655013 THRU 2498135 | |
| Mercury | 40 (1 CYL.) | 4 | 27-89937 | Serial #: 2498136 THRU 3296136 |
Technical Specs
- Product Type
- Powerhead Base Gasket
- Installation
- Mounts between powerhead and adapter plate
- Housing Material
- OEM Specification Material
- Dimensions
- 9.31 in x 6.0 in x 1.13 in
- Warranty
- Manufacturer standard parts warranty
What’s in the Box
- 1 x Powerhead Base Gasket (27-89937)
Installation Notes
- Ensure the engine is off and cool, and disconnect the battery.
- Remove the engine cowling to access the powerhead.
- Carefully unbolt and lift the powerhead from the adapter plate.
- Remove the old gasket and thoroughly clean both mating surfaces of any residue.
- Position the new gasket, ensuring all bolt holes are correctly aligned.
- Reinstall the powerhead, tightening bolts in a star pattern to the torque specified in your service manual.
- Reconnect all components and test the engine in a safe environment before full operation.
Troubleshooting & FAQ
How do I know if this gasket fits my engine?
Refer to the Engine & Model Compatibility table. It is crucial to match your engine's model, horsepower, and particularly the serial number range to confirm correct fitment.
What are the signs of a failed powerhead base gasket?
Common symptoms include loss of engine compression, water intrusion into the cylinders, engine overheating, or a visible leak (water or exhaust) from the seam between the powerhead and the adapter plate.
Do I need to use gasket sealant with this part?
In most cases, this gasket is designed for a dry installation. However, always consult your engine's official service manual for specific recommendations regarding the use of sealants.
Is this part for a 2-stroke or 4-stroke engine?
This gasket is designed for a range of single-cylinder Mercury/Mariner outboards from the mid-1970s to mid-1980s, which are typically 2-stroke engines. Always verify compatibility with your specific model details.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.
Shipping speed & cost
We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.
Safety & Important Info
- Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement, not an OEM component.
- Professional installation is recommended for safety and performance.
About Sea Sierra
- Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
- Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
- Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.
Payment & Security
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