Lower Unit Gear Box Gasket Kit – Fits Yamaha & Mariner 9.9HP/15HP (Replaces 683-W0001-21-00)
Lower Unit Gear Box Gasket Kit – Fits Yamaha & Mariner 9.9HP/15HP (Replaces 683-W0001-21-00) 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
Lower Unit Gear Box Gasket Kit – Fits Yamaha & Mariner 9.9HP/15...
Product Details
This Lower Unit Gear Box Gasket Kit provides the necessary components to service the gearcase on a range of Yamaha and Mariner outboard motors. It is designed for 9.9HP and 15HP 2-stroke and 4-stroke models manufactured between 1985 and 2004.
The kit serves as a direct aftermarket replacement for Yamaha part numbers 683-W0001-21-00 and 683-W0001-C1-00, Mercury 27-99049M, and Sierra 18-74501.
- Enables a complete, direct-replacement installation.
- Includes all essential gaskets and seals for one lower unit service.
- Manufactured in Taiwan to meet or exceed OEM specifications.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket suppliers.
| Brand | Part Number | Superseded By |
|---|---|---|
| Yamaha | 683-W0001-21-00 | 683-W0001-C1-00 |
| Mercury | 27-99049M / 99049M | |
| Sierra | 18-74501 |
Engine & Model Compatibility
Confirm your engine model and year in the tables below.
Yamaha Fitment: This kit is compatible with 9.9HP and 15HP 2-stroke and 4-stroke models from 1985–2004.
Mariner Fitment: This kit is compatible with 9.9HP and 15HP 2-stroke models from 1985–1994.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | 9.9C/D | 9.9 | 2-Stroke | 1985–1994 | 683-W0001-21-00 | Superseded by 683-W0001-C1-00 |
| Yamaha | 15C/D | 15 | 2-Stroke | 1985–1994 | 683-W0001-21-00 | Superseded by 683-W0001-C1-00 |
| Yamaha | F9.9A/B | 9.9 | 4-Stroke | 1985–2004 | 683-W0001-21-00 | Superseded by 683-W0001-C1-00 |
| Yamaha | FT9.9A/B | 9.9 | 4-Stroke | 1985–2004 | 683-W0001-21-00 | Superseded by 683-W0001-C1-00 |
| Mariner | 9.9C/D | 9.9 | 2-Stroke | 1985–1994 | 27-99049M | Also listed as 99049M |
| Mariner | 15C/D | 15 | 2-Stroke | 1985–1994 | 27-99049M | Also listed as 99049M |
Technical Specs
- Product Type
- Lower Unit/Gear Box Gasket Kit
- Installation
- Direct replacement
- Weight
- 50g to 156g (0.11 lb to 0.34 lb)
- Product Dimensions
- 7.0" L x 6.0" W x 0.5" H
- Country of Origin
- Taiwan
What’s in the Box
- Complete set of gaskets and seals for one lower unit service.
Installation Notes
- Position the outboard securely on a stand or with the engine tilted up.
- Place a drain pan under the lower unit and remove the lower and upper gearcase oil drain/fill screw plugs. Allow the oil to drain completely.
- Remove the mounting bolts that secure the lower unit to the midsection. Keep track of bolt lengths and positions.
- Carefully separate the lower unit from the engine. You may need to disconnect the shift linkage.
- Remove the old gaskets and seals from the mating surfaces. Use a plastic scraper to avoid gouging the aluminum housing.
- Thoroughly clean all mating surfaces with a suitable solvent to remove any old sealant and residue.
- Install the new gaskets and seals from the kit onto the appropriate surfaces.
- Reattach the lower unit, ensuring the driveshaft, water tube, and shift shaft align correctly.
- Reinstall the mounting bolts and torque them to the manufacturer's specification.
- Refill the gearcase with new marine gear lubricant through the bottom hole until it flows out the top hole, then reinstall both screw plugs with new washers.
Troubleshooting & FAQ
How do I confirm this kit fits my specific engine?
The best way is to cross-reference your engine's model, year, and original part number with the information in the 'Engine & Model Compatibility' and 'Part Number Matrix' sections. Always check the OEM part number from your engine's service manual against our list.
What are the signs my lower unit gaskets are failing?
Common signs include a milky or cloudy appearance in your gear lube (indicating water intrusion), visible oil leaks around the gearcase seams, or a metallic sheen in the oil from gear wear caused by water contamination.
How often should lower unit gaskets be replaced?
There is no fixed schedule, but gaskets should be replaced whenever the lower unit is removed for service, such as during a water pump impeller replacement. It is also wise to replace them if you notice any signs of leaks or water intrusion.
Does this kit include every seal in the lower unit?
This kit is designed to replace all the primary flat gaskets and seals required when separating and re-installing the lower gearcase housing. It typically includes water pump gaskets and gear housing seals but may not include internal shaft seals unless specified.
What is the difference between OEM and aftermarket parts?
This listing is for an aftermarket replacement, not an OEM component. Our parts are engineered to meet or exceed OEM-level specifications using comparable materials. They are manufactured in an ISO 9001 certified facility and are batch-inspected to ensure quality and performance.
How fast is shipping?
We ship worldwide, with most orders arriving in 5–15 days via standard shipping. Expedited options are available at checkout for certain regions. Standard shipping is free on all orders.
What is your warranty and return policy?
This product is covered by a 1-year warranty against manufacturing defects under normal marine use. If you encounter an issue, please contact our support team with your order number. Returns are accepted within our standard policy window for items that are unused and in their original packaging. Please see our policy page or contact us for instructions.
Safety & Important Info
- Verify images and specifications against your original part before purchase. If anything differs—even if your model appears on the compatibility list—do not order.
- This is an aftermarket replacement part, not an Original Equipment Manufacturer (OEM) component.
- Professional installation is highly recommended for safety, proper function, and to prevent damage.
About Sea Sierra
- Pro-Grade Standards: We supply professional-grade marine parts engineered to OEM-level tolerances and rigorously tested for real-world reliability.
- Fast Global Fulfillment: We provide fast, free standard delivery to most regions, with typical transit times of 5–15 days.
- Manufacturer-Direct Value: By working directly with manufacturers, we offer fair, transparent pricing without compromising on materials or quality control.
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