Piston Kit (STD) – Fits Yamaha 25-50HP 2-Stroke Outboards (Replaces 6H4-11631-01-95)
Piston Kit (STD) – Fits Yamaha 25-50HP 2-Stroke Outboards (Replaces 6H4-11631-01-95) 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 is a standard bore Piston Kit (STD) designed for a range of Yamaha 2-Stroke outboard motors, including 25HP, 40HP, and 50HP models manufactured from 1989 to the present. The kit is also a direct fit for Yamaha C40 models (1997–2003) and C50 models (1998–2001).
It serves as a direct replacement for Yamaha OEM part numbers 6H4-11631-01-95, 6H4-11631-09-96, and 6H4-11631-00-96.
- Installation: Mounts within the engine block.
- Engine Compatibility: For gasoline-powered 2-stroke outboards.
- Complete Kit: Includes the piston, rings, wrist pin, and circlips for one cylinder.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket suppliers.
| Brand | Part Number |
|---|---|
| Yamaha | 6H4-11631-01-95 |
| Yamaha | 6H4-11631-09-96 |
| Yamaha | 6H4-11631-00-96 |
Engine & Model Compatibility
Confirm your engine model and year in the table below to ensure accurate fitment.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | 25Q/QEO | 25 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40A/C40/ELR | 40 | 2-Stroke | 1989–2003 | 6H4-11631-01-95 | |
| Yamaha | 40H/HE | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40MLHZ | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40O/OS | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40TLR | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40V/VE/VEO/VETO/VMHO | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 40YETO/Z | 40 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 50D/DE | 50 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 50H/HEDO/HETO/HMHD/HWHDLO/HRDO | 50 | 2-Stroke | 1989–Current | 6H4-11631-01-95 | |
| Yamaha | 50MTO | 50 | 2-Stroke | 1989–Current | 6H4-11631-01-95 |
Technical Specs
- Product Type
- Piston Kit
- Installation
- Engine Block
- Fuel Type
- Gasoline (2-Stroke)
- Housing Material
- Cast Aluminum
- Standard Bore
- 67mm (2.638")
- Piston Height
- 72.41mm
- Wrist Pin Diameter
- 18mm
- Ring Configuration
- 2-ring design
- Product Dimensions
- 9.0" H x 9.0" W x 9.0" L
What’s in the Box
- 1 x Piston (STD) (Replaces 6H4-11631-01-95)
- 1 x Piston Ring Set (Standard)
- 1 x Wrist Pin
- 2 x Circlips
Installation Notes
Note: This kit is designed for engines with 2-ring pistons. It may be used to service older models (e.g., 1984–1988) originally equipped with 3-ring pistons, but only if all pistons in the engine are replaced as a set to ensure balanced operation.
- Disconnect the battery and secure fuel lines.
- Drain engine oil and coolant completely.
- Remove the cylinder head cover, gaskets, and any components obstructing access.
- Carefully push the old piston out from the connecting rod after removing the cylinder.
- Inspect the cylinder wall for scratches or scoring. Hone if necessary to ensure a proper seal.
- Assemble the new piston, wrist pin, and circlips onto the connecting rod.
- Correctly orient and gap the new piston rings according to the service manual.
- Use a ring compressor to gently install the piston assembly into the cylinder.
- Reassemble the engine using new gaskets and torque all fasteners to OEM specifications.
- Refill oil and coolant, then inspect for leaks before starting the engine.
Troubleshooting & FAQ
Will this piston fit my older Yamaha model with 3-ring pistons (pre-1989)?
Yes, this 2-ring piston kit can be used in 1984–1988 models, but you must replace the pistons in all cylinders at the same time. This ensures a balanced engine and is critical for performance and longevity. This kit is not a direct single-piston swap for original 3-ring designs.
Is this a standard size piston? Are oversized options offered?
Yes, this is a standard bore (STD) piston kit with a 67mm bore. Oversized versions (e.g., 0.25mm, 0.50mm) are available to accommodate rebored cylinders but are sold as separate products. This listing is for the standard size only.
What are common symptoms of a failing outboard piston?
Signs that a piston needs replacement include a noticeable loss of engine compression, excessive blue or white smoke from the exhaust, a metallic knocking or slapping sound from the engine block, and a general decrease in power and performance.
What’s the difference between this 2-ring design and a 3-ring design?
A 2-ring piston creates less friction, which can lead to slightly better high-RPM performance. A 3-ring piston generally offers improved oil control and a more robust seal over time. This particular kit is engineered as a direct replacement for OEM 2-ring pistons found in the specified models.
OEM vs. Aftermarket?
This listing is an aftermarket replacement (not OEM). It is built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, it is batch-inspected for consistency and reliability.
Shipping speed & cost
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
Warranty & returns
This product is 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 for unused items in original condition. See our policy page or reach out for instructions.
Safety & Important Info
- Verify images and specifications against your original part before purchase. If anything differs—even if your model is listed as compatible—do not order.
- This is an aftermarket replacement part, not an OEM component. Brand names and part numbers are used for reference purposes only.
- Professional installation is strongly recommended for safety, proper function, and to prevent engine damage.
About Sea Sierra
- Pro-Grade Standards: We supply marine parts engineered to OEM-level tolerances and rigorously tested for real-world conditions.
- Fast Global Fulfillment: We offer free standard delivery to most regions, with an average transit time of 5–15 days.
- Manufacturer-Direct Value: Our parts provide fair pricing without compromising on materials or quality control.
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