Mercruiser Piston Kit – Fits Mercury Marine 3.375" Bore Engines (Replaces 777-815965A6)
Mercruiser Piston Kit – Fits Mercury Marine 3.375" Bore Engines (Replaces 777-815965A6) 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
Mercruiser Piston Kit – Fits Mercury Marine 3.375" Bore Engine...
Product Details
This Mercruiser Piston Kit is engineered for Mercury Marine 3-cylinder and 4-cylinder 3.375" bore looper engines, including 70–115 HP 2-stroke models manufactured between 1985 and 1993. It's designed for standard bore engines that have been bored 0.030" over.
This kit provides a direct interchange for Sierra 18-4625 and Quicksilver 777-815965A6, ensuring a precise fit and reliable performance.
- Built to match OEM specifications for durability and function.
- Engineered for standard 3.375" bore engines machined 0.030" oversize.
- Includes the piston assembly required for a complete replacement.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket brands.
| Brand | Part Number |
|---|---|
| Sierra | 18-4625 |
| Quicksilver | 777-815965A6 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Mercury Engines: This piston kit fits specific 3-cylinder and 4-cylinder looper models from 70 HP to 115 HP made between 1985 and 1993.
For Mariner Engines: Compatibility is confirmed for all models that specify the use of part number 18-4625.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|---|
| Mercury | 3 Cyl 3.375 Bore Looper | 70 | 2-Stroke | 1987–1990 | 777-815965A6 |
| Mercury | 3 Cyl 3.375 Bore Looper | 75 | 2-Stroke | 1987–1993 | 777-815965A6 |
| Mercury | 3 Cyl 3.375 Bore Looper | 80 | 2-Stroke | 1987–1990 | 777-815965A6 |
| Mercury | 3 Cyl 3.375 Bore Looper | 90 | 2-Stroke | 1987–1993 | 777-815965A6 |
| Mercury | 4 Cyl 3.375 Bore Looper | 100 | 2-Stroke | 1985–1993 | 777-815965A6 |
| Mercury | 4 Cyl 3.375 Bore Looper | 115 | 2-Stroke | 1989–1993 | 777-815965A6 |
| Mariner | All models with part number 18-4625 | 2-Stroke | 18-4625 |
Technical Specs
- Product Type
- Piston Kit
- Bore Size
- 3.672" (93.27 mm)
- Bore Oversize
- 0.030" Over
- Application
- Fits 3.375" standard bore engines bored 0.030" over
- Dimensions
- 8.0" x 9.0" x 9.5"
- Weight
- 1.2 lb (0.54 kg)
- Warranty
- One Year Manufacturer's Warranty
What’s in the Box
- 1 x Piston Assembly (Replaces 777-815965A6)
Installation Notes
- Disconnect the battery and ensure the engine is cool before starting.
- Remove the cylinder head and any components obstructing access to the pistons.
- Carefully remove the old piston, connecting rod, and associated bearings.
- Clean the cylinder walls and connecting rod thoroughly, inspecting for wear or damage.
- Properly lubricate the new piston, rings, and wrist pin with engine assembly lube.
- Install the new piston assembly, ensuring rings are correctly clocked and aligned.
- Reassemble all components using the correct torque specifications.
- After reassembly, manually turn the engine to check for binding before starting.
Troubleshooting & FAQ
How do I confirm this piston kit fits my engine?
Verify your engine model, horsepower, and year against the compatibility table provided. It's also critical to cross-reference the OEM part number (777-815965A6 or 18-4625) listed in your service manual or on the original part.
What does 0.030" oversize mean?
This piston is intended for an engine block where the cylinders have been machined (bored) 0.030 inches larger than the standard diameter to repair wear or damage. It will not fit a standard, un-bored cylinder.
What are the signs of a failed piston?
Common symptoms include loss of compression in one or more cylinders, excessive blue or white smoke from the exhaust, a knocking or rattling sound from the engine, and metal shavings in the engine oil.
Can I replace just one piston?
While possible, it is generally recommended to replace pistons as a complete set to ensure balanced engine performance, weight, and compression across all cylinders.
Is any special machining required for installation?
Yes, your engine cylinder must be professionally bored and honed to precisely match the 0.030" oversize of this piston kit for proper fit, seal, and performance.
What is the difference between this and the OEM part?
This is an aftermarket part designed to meet or exceed original equipment manufacturer (OEM) specifications for fit, material, and performance. It offers a cost-effective alternative to OEM parts without compromising quality.
What is the recommended maintenance interval?
Pistons do not have a fixed replacement interval and are typically replaced only during a complete engine rebuild or when they fail. Proper engine maintenance, including regular oil changes and using correct fuel, will extend their life.
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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