Lower Unit Gasket Kit – Fits Yamaha 40HP/50HP 2-Stroke (Replaces 6H4-W0001-21-00, 6H4-W0001-20-00)
Lower Unit Gasket Kit – Fits Yamaha 40HP/50HP 2-Stroke (Replaces 6H4-W0001-21-00, 6H4-W0001-20-00) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
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.).
You should expect to receive your refund within four weeks of giving your package to the return shipper, however, in many cases you will receive a refund more quickly. This time period includes the transit time for us to receive your return from the shipper (5 to 10 business days), the time it takes us to process your return once we receive it (3 to 5 business days), and the time it takes your bank to process our refund request (5 to 10 business days).
If you need to return an item, simply login to your account, view the order using the "Complete Orders" link under the My Account menu and click the Return Item(s) button. We'll notify you via e-mail of your refund once we've received and processed the returned item.
Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Product Details
This Lower Unit Gasket Kit for Yamaha 40HP/50HP 2-Stroke is a complete gearcase seal kit for outboard engines manufactured from 1984 to 1994. It contains all the necessary gaskets, seals, and O-rings for comprehensive lower unit service and repair.
This kit provides a direct replacement for Yamaha part numbers 6H4-W0001-21-00, 6H4-W0001-20-00, 6H4-W0001-21, 6H4-W0001-C0-00, 6H4-W0001-C0, and Sierra 18-2792.
- Contains all essential seals, gaskets, and O-rings for a complete lower unit rebuild.
- Engineered for Yamaha's 3-Cylinder 40HP and 50HP 2-stroke outboards.
- Professional installation is recommended for optimal sealing and performance.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket suppliers.
| Brand | Part Number |
|---|---|
| Yamaha | 6H4-W0001-21-00 |
| Yamaha | 6H4-W0001-20-00 |
| Yamaha | 6H4-W0001-21 |
| Yamaha | 6H4-W0001-C0-00 |
| Yamaha | 6H4-W0001-C0 |
| Sierra | 18-2792 |
Engine & Model Compatibility
Confirm your engine model and year in the table below.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | 40HP (3-Cylinder) | 40 | 2-Stroke | 1984–1994 | 6H4-W0001-21-00 | Includes models 40A, C40, 40ELR, 40H, 40HE, 40MLHZ, 40O, 40OS, 40TLR, 40V, 40VE, 40VEO, 40VETO, 40VMHO, 40YETO, 40Z |
| Yamaha | 50HP (3-Cylinder) | 50 | 2-Stroke | 1984–1994 | 6H4-W0001-21-00 | Includes models 50D, 50DE, 50H, 50HEDO, 50HETO, 50HMHD, 50HWHDLO, 50HRDO, 50MTO |
Technical Specs
- Product Type
- Lower Unit/Gearcase Seal Kit
- Installation
- Professional installation recommended
- Applications
- Lower unit/gearcase assembly
- Engine Type
- 2-Stroke outboards
- Contents
- Complete seal kit for gearcase rebuild
- OEM Compatibility
- Yamaha 40HP/50HP (1984–1994)
- Other Technical Attributes
- Contains all necessary seals, gaskets, O-rings for complete lower unit rebuild
- Package Dimensions
- 12.0" x 7.0" x 1.25"
What’s in the Box
- Complete gasket and seal kit for lower unit rebuild.
Installation Notes
- Drain the gearcase oil completely before beginning disassembly.
- Carefully detach the lower unit from the outboard's midsection, disconnecting any linkage.
- Disassemble the gearcase and driveshaft housing, keeping all components organized.
- Thoroughly clean all mating surfaces to remove old gasket material and sealant residue.
- Systematically replace all old seals, O-rings, and gaskets with the new components from the kit.
- Lightly lubricate new seals and O-rings with marine grease before installation to prevent damage.
- Reassemble the gearcase according to your engine's service manual specifications, ensuring correct alignment.
- Re-install the lower unit, torquing all bolts to the manufacturer's specified values.
- Refill the gearcase with the correct type and amount of fresh lower unit oil.
- Before operating, check for any oil or water leaks around the newly sealed areas.
Troubleshooting & FAQ
How do I know if this gasket kit fits my Yamaha engine?
This kit is specifically designed for Yamaha 40HP and 50HP 3-cylinder 2-stroke outboards manufactured from 1984 to 1994. To ensure a perfect fit, cross-reference your engine's model number and year with the detailed compatibility table on this page.
What are the signs of a failing lower unit gasket?
The most common indicator is milky, grey, or water-contaminated gear oil, which means water is entering the gearcase. Other signs include visible oil leaks around the prop shaft or gearcase seams and, in severe cases, a burning smell from the lower unit.
Can I just replace the one seal that is leaking?
While technically possible, it is strongly recommended to replace all seals and gaskets at once. The other seals are of the same age and have been exposed to the same conditions, meaning they are likely to fail soon. Replacing the full kit prevents repeating the labor-intensive disassembly process.
Is marine sealant required in addition to the gaskets?
Yes, for certain mating surfaces, applying a thin layer of marine-grade gasket sealant is recommended to ensure a durable, long-lasting seal. Always consult your engine's service manual for specific sealant types and application points.
Does this kit include everything I need for the job?
This kit contains all the necessary gaskets, seals, and O-rings for a complete lower unit gearcase rebuild. However, you will still need tools, marine grease, and gear lube. Professional installation is recommended.
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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