Piston (0.25mm Oversize) – Fits Yamaha 25–50 HP (1990–2003) (Replaces 6H4-11635-01-00)
Piston (0.25mm Oversize) – Fits Yamaha 25–50 HP (1990–2003) (Replaces 6H4-11635-01-00) 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 high-quality Piston (0.25mm Oversize) – Fits Yamaha 25–50 HP (1990–2003) is engineered for 2-stroke Yamaha outboard engines from 25 to 50 HP, covering models from approximately 1990 to 2003. It is designed for cylinders that have been bored to 0.25mm oversize and is compatible with a wide range of C40, 40V, 50H, and other series engines.
It serves as a direct interchange for Yamaha part numbers 6H4-11635-01-00 and 6H4-11635-01, as well as aftermarket equivalents like Sierra 100-252-04K.
- Complete kit includes the piston, rings, wrist pin, and clips for a full replacement.
- Constructed from durable cast aluminum for reliable performance in marine environments.
- Installs in the engine block assembly of compatible gasoline (2-Stroke) outboards.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Yamaha | 6H4-11635-01-00 |
| Yamaha | 6H4-11635-01 |
| Sierra | 100-252-04K |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This piston kit fits the following Yamaha outboard models. Please verify your specific model and year before ordering.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Yamaha | 25Q/QEO | 25 | 2-Stroke | 1990–1994 | 6H4-11635-01-00 | |
| Yamaha | 40A | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | C40 | 40 | 2-Stroke | 1997–2003 | 6H4-11635-01-00 | |
| Yamaha | 40ELR | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40H/HE | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40MLHZ | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40O/OS | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40TLR | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40V/VE/VEO/VETO/VMHO | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 40YETO/Z | 40 | 2-Stroke | 1989–1997 | 6H4-11635-01-00 | |
| Yamaha | 50D/DE | 50 | 2-Stroke | 1989–1994 | 6H4-11635-01-00 | |
| Yamaha | C50 | 50 | 2-Stroke | 1998–2001 | 6H4-11635-01-00 | |
| Yamaha | 50H/HEDO/HETO/HMHD/HWHDLO/HRDO | 50 | 2-Stroke | 1989–1994 | 6H4-11635-01-00 | |
| Yamaha | 50MTO | 50 | 2-Stroke | 1989–1994 | 6H4-11635-01-00 |
Technical Specs
- Product Type
- Piston (0.25mm Oversize)
- Installation
- Engine Block Assembly
- Fuel Type
- Gasoline (2-Stroke)
- Material
- Cast Aluminum
- Diameter
- 2.648" / 67.26mm (0.25mm/0.010" Oversize)
- Bore Size
- 0.25mm Oversize (0.010" Oversize)
- Standard Bore
- 2.638" / 67.01mm
- Ring Configuration
- 2-Ring Design
- Kit Contents
- Complete kit includes piston, rings, wrist pin and clips
- Product Dimensions
- 3.5" L x 3.0" W x 3.0" H
- Weight
- 0.35 kg (0.77 lb)
What’s in the Box
- 1x Piston (0.25mm Oversize) (Replaces 6H4-11635-01-00)
- 1x Piston Ring Set
- 1x Wrist Pin
- 2x Circlips
Installation Notes
- Ensure the engine is off and cool. Disconnect the battery and fuel supply for safety.
- Drain the engine oil and coolant completely.
- Carefully remove the cylinder head, gasket, and any interfering components.
- Rotate the crankshaft to bring the target piston to Bottom Dead Center (BDC).
- Unbolt the connecting rod cap from below and gently remove the bearing half.
- From the top of the block, carefully push the piston and rod assembly out of the cylinder bore.
- Remove the old circlips and wrist pin to separate the old piston from the connecting rod.
- Assemble the new piston onto the connecting rod using the new wrist pin and circlips, ensuring correct orientation.
- Using a piston ring compressor, carefully guide the new piston and ring assembly back into the cylinder bore.
- Re-attach the connecting rod cap with its bearing, and torque the bolts to the manufacturer's specification.
- Install a new head gasket and re-install the cylinder head, torquing bolts in the correct sequence.
- Re-assemble all other components, refill with fresh fluids, and perform a break-in procedure as recommended by the engine manufacturer.
Troubleshooting & FAQ
How do I know if this 0.25mm oversize piston is correct for my engine?
This oversize piston is designed for engine cylinders that have been machined or bored 0.25mm (or 0.010 inches) larger than their standard diameter. This is a common procedure during an engine rebuild to repair worn or damaged cylinder walls. You must measure your cylinder bore to confirm it matches this oversize dimension before purchasing.
What are the signs that my old piston is failing?
Common symptoms of piston failure include a noticeable loss of engine compression, an increase in oil consumption, unusual knocking or slapping sounds from the engine, and excessive blue or white smoke from the exhaust.
Does this kit include everything needed for the piston installation?
Yes, this is a complete kit containing the piston, a full set of piston rings, the wrist pin, and two circlips. You will still need gaskets, seals, and appropriate lubricants, which are typically sold separately.
What is the proper break-in procedure for a new piston?
For optimal performance and longevity, a new piston and rings require a break-in period. Generally, this involves running the engine at varied, low-to-mid RPMs for the first 5-10 hours of operation. Avoid sustained high speeds or heavy loads, as this allows the new rings to seat properly against the cylinder wall.
How does this piston's 2-ring design work?
A 2-ring configuration is typical for many 2-stroke outboard engines. The top ring is primarily for compression sealing, while the second ring assists with compression and also helps scrape excess oil from the cylinder wall to prevent it from burning in the combustion chamber. This design provides an effective balance of sealing, durability, and low friction.
Is this an OEM part?
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.
How fast is shipping?
We ship worldwide quickly—typical transit is 5–15 days. Standard shipping is free; expedited options appear at checkout where available.
What is the warranty and return policy?
This part 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 (the part must be unused and in its 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.
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