Lower Unit Seal Kit – Fits Mercury/Mariner 9.9-25HP Outboards (Replaces 26-85090A2, 18-2624)
Lower Unit Seal Kit – Fits Mercury/Mariner 9.9-25HP Outboards (Replaces 26-85090A2, 18-2624) is backordered and will ship as soon as it is back in stock.
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Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
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Lower Unit Seal Kit – Fits Mercury/Mariner 9.9-25HP Outboards (Repl...
Lower Unit Seal Kit – Fits Mercury/Mariner 9.9-25HP Outboards (Replaces 26-85090A2, 18-2624) is backordered and will ship as soon as it is back in stock.
Product Details
This Lower Unit Seal Kit provides the essential components to service and reseal the lower gearcase on Mercury/Mariner 9.9–25HP outboard engines. Designed for both 2-Stroke and 4-Stroke models, this kit helps prevent water intrusion and gear lube leaks, restoring the integrity of your outboard's lower unit.
It serves as a direct aftermarket replacement for Mercury OEM part 26-85090A2 and interchanges with Sierra 18-2624 and SeaSierra 18-2624.
- Engineered for direct replacement of the original seals.
- Includes a comprehensive set of seals, O-rings, and gaskets for a complete service.
- Constructed from high-quality, reinforced rubber and gasket materials for durability and a reliable seal.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket providers.
| Brand | Part Number |
|---|---|
| Mercury | 26-85090A2 |
| Sierra | 18-2624 |
| SeaSierra | 18-2624 |
Engine & Model Compatibility
Confirm your engine model and horsepower in the table below. Matching the OEM part number is the most reliable way to ensure correct fitment.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | Outboard | 9.9 | 2-Stroke & 4-Stroke | 26-85090A2 | ||
| Mercury | Outboard | 15 | 2-Stroke & 4-Stroke | 26-85090A2 | ||
| Mercury | Outboard | 20 | 2-Stroke | 26-85090A2 | ||
| Mercury | Outboard | 25 | 2-Stroke | 26-85090A2 | ||
| Mariner | Outboard | 9.9 | 2-Stroke & 4-Stroke | 26-85090A2 | ||
| Mariner | Outboard | 15 | 2-Stroke & 4-Stroke | 26-85090A2 | ||
| Mariner | Outboard | 20 | 2-Stroke | 26-85090A2 | ||
| Mariner | Outboard | 25 | 2-Stroke | 26-85090A2 |
Technical Specs
- Product Type
- Lower Unit Seal Kit
- Installation
- Direct replacement
- Housing Material
- Reinforced rubber seals and gaskets
- Housing Color
- Black
- Weight
- 50g (0.11 lb)
What’s in the Box
- 1 x Lower Unit Seal Kit (Replaces 26-85090A2 / 18-2624)
- Assorted seals, gaskets, and O-rings for complete lower unit service
Installation Notes
- Drain the gearcase oil completely from the lower unit.
- Remove the propeller, thrust washer, and any related hardware.
- Unbolt and carefully separate the lower unit from the outboard's midsection.
- Disassemble the water pump assembly to gain access to the driveshaft seals.
- Disassemble the propeller shaft bearing carrier to access the propeller shaft seals.
- Carefully remove all old seals, o-rings, and gaskets. Using a seal puller is recommended to prevent damage to the housing.
- Thoroughly clean all sealing surfaces, removing any old adhesive or sealant.
- Lightly lubricate new seals and O-rings with marine grease before installation.
- Install the new seals squarely into their bores using a suitable seal driver or socket.
- Reassemble all components in the reverse order of disassembly, ensuring bolts are torqued to manufacturer specifications.
- Install a new drain screw washer and refill the gearcase with the correct type and amount of fresh gear lube.
- Submerge the lower unit and check for any signs of water intrusion or oil leaks before operating.
Troubleshooting & FAQ
How do I know if this seal kit fits my specific engine year?
This kit is designed for a wide range of Mercury/Mariner 9.9HP, 15HP, 20HP, and 25HP outboards. Since specific year ranges are not always provided by manufacturers, the best method is to match the OEM part number (26-85090A2) from your engine's service manual with the one this kit replaces.
What are the signs that my lower unit seals are failing?
Common signs include a milky or foamy appearance in your gear lube (indicating water has mixed with the oil) or a visible oil leak around the propeller shaft or driveshaft housing. Fishing line wrapped around the prop shaft is a frequent cause of seal failure.
How often should lower unit seals be replaced?
There is no fixed replacement interval. It is best practice to inspect for leaks at every service. We recommend proactively replacing the seals whenever the lower unit is disassembled for other maintenance (like a water pump impeller change) or immediately if you detect a leak.
Is any special tool required for installation?
While basic hand tools are sufficient for disassembly, using a dedicated seal puller for removal and a proper seal driver (or a socket of the correct diameter) for installation is highly recommended to avoid damaging the new seals or the housing surfaces.
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 and specifications against your original part. If anything differs—even if your model appears compatible—do not purchase.
- This is an aftermarket replacement component, not an OEM part.
- Professional installation is recommended for safety and optimal 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 quality control.
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