Piston Kit – Fits Mercury/Mariner 70-115 HP (Replaces 777-815965A4)
Piston Kit – Fits Mercury/Mariner 70-115 HP (Replaces 777-815965A4) is backordered and will ship as soon as it is back in stock.
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Worry-free returns
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Delivery and Shipping
Delivery and Shipping
Free standard shipping on all orders under 1 KG
Piston Kit – Fits Mercury/Mariner 70-115 HP (Replaces 777-815965A4)
Product Details
This top-guided Piston Kit – Fits Mercury/Mariner 70-115 HP (Replaces 777-815965A4) is engineered for 3 and 4-cylinder looper engines from Mercury and Mariner, covering models from 1987–1993. It is designed to restore compression and performance in specified powerheads.
It serves as a direct interchange for Mercury/Mariner part numbers 777-815965A4 and 815965T4, along with aftermarket cross-references including WSM 100-35K and Sierra 18-4227.
- Installation: Mounts in the engine powerhead/cylinder block.
- Piston Design: Features a top-guided design required for specific Mercury/Mariner looper engines.
- Fuel Type: Compatible with gasoline-fueled 2-stroke outboards.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number |
|---|---|
| Mercury/Mariner | 777-815965A4 |
| Mercury/Mariner | 815965T4 |
| WSM | 100-35K |
| Sierra | 18-4227 |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
For Mercury/Mariner Engines: Please ensure your model is a 3 or 4-cylinder looper design with a 3.375" standard bore, as specified for these years.
| Brand | Model | Rated HP | Stroke | Years | OEM Part No. | Notes |
|---|---|---|---|---|---|---|
| Mercury | 3 Cylinder Looper | 70 | 2-Stroke | 1987–1990 | 777-815965A4 | 3.375" Bore |
| Mercury | 3 Cylinder Looper | 75 | 2-Stroke | 1992–1993 | 777-815965A4 | 3.375" Bore |
| Mercury | 3 Cylinder Looper | 80 | 2-Stroke | 1987–1990 | 777-815965A4 | 3.375" Bore |
| Mercury | 3 Cylinder Looper | 90 | 2-Stroke | 1992–1993 | 777-815965A4 | 3.375" Bore |
| Mercury | 4 Cylinder Looper | 100 | 2-Stroke | 1992–1993 | 777-815965A4 | 3.375" Bore |
| Mercury | 4 Cylinder Looper | 115 | 2-Stroke | 1992–1993 | 777-815965A4 | 3.375" Bore |
Technical Specs
- Product Type
- Top-Guided Piston Kit
- Application
- Engine Powerhead/Cylinder Block
- Housing Material
- Aluminum Alloy
- Bore Size
- 3.375" (85.73 mm)
- Design
- Top-guided piston design for Mercury looper engines
- Product Dimensions
- 4.25" x 4.25" x 4.25"
What’s in the Box
- 1 x Piston Kit (Replaces 777-815965A4)
Installation Notes
- Disconnect the battery and ensure the engine is cool. Relieve any pressure from the fuel system.
- Remove the powerhead cover, cylinder head, and any other components obstructing access to the pistons.
- Carefully rotate the crankshaft to bring the piston you are replacing to the bottom of its stroke.
- Remove the connecting rod cap and gently push the old piston assembly out from the top of the cylinder.
- Thoroughly clean the cylinder walls and inspect for wear, scoring, or damage. Hone the cylinder as needed for proper ring seating.
- Assemble the new piston, wrist pin, and circlips onto the connecting rod, ensuring correct orientation.
- Use a ring compressor tool to carefully install the new piston and ring assembly into the cylinder.
- Lubricate all components with fresh engine oil or assembly lube during installation.
- Reinstall the connecting rod cap to the manufacturer's specified torque.
- Reassemble the cylinder head with a new gasket and torque all bolts in the correct sequence.
- Complete reassembly, reconnect the battery, and prime the fuel system.
- Follow the recommended engine break-in procedure for new piston components to ensure longevity and performance.
Troubleshooting & FAQ
How do I confirm this piston fits my engine?
This kit fits specific Mercury/Mariner 70-115 HP looper engines from 1987–1993. Check your engine model and year against the compatibility chart. The standard bore size for this kit is 3.375 inches; measure your cylinder to confirm before ordering.
What are the signs of a failing piston?
Common symptoms include low compression in one or more cylinders, excessive blue or white smoke from the exhaust, a metallic knocking or rattling sound from the engine block, and a noticeable loss of power or poor idle.
Does this kit include piston rings, a wrist pin, and circlips?
Yes, a piston kit traditionally includes the piston, piston rings, wrist pin, and circlips. Please refer to the "What’s in the Box" section and product images to confirm the exact contents.
Why is a "top-guided" piston important for my engine?
Top-guided pistons are required for certain engine designs, like Mercury's looper models, to maintain stability and alignment within the cylinder. Using a non-guided piston in an engine that requires one can lead to premature wear and catastrophic failure.
Do I need to hone the cylinder before installing a new piston?
It is strongly recommended. Honing the cylinder creates a crosshatch pattern that helps the new piston rings seat correctly, ensuring proper sealing, compression, and oil control. Skipping this step can result in poor performance and reduced engine life.
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.
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