Piston Kit – Fits Mercury/MerCruiser 135-200 HP V6 Outboards (Replaces 765-7445A12)

$102.90 $164.00

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Product Details

This aftermarket Piston Kit – Fits Mercury/MerCruiser 135-200 HP V6 Outboards (Replaces 765-7445A12) is designed for Mercury and Mariner 135–200 HP V6 2-Stroke outboard engines with a 3.125" standard bore, manufactured between 1976 and 1991. It provides a reliable replacement for restoring engine compression and performance.

This kit serves as a direct interchange for Mercury 765-7445A12, WSM 100-50-06SK, and Sierra 18-4015.

Installs on the starboard (right) side of the engine block.
Features a bottom-guided design for use with connecting rod #636.
Machined to .030" oversize for cylinders requiring re-boring.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket.

Brand	Part Number
Mercury	765-7445A12
WSM	100-50-06SK
Sierra	18-4015

Engine & Model Compatibility

Confirm your engine model/year in the table below.

This piston kit is designed for select Mercury and Mariner V6 outboards. Ensure your specific model and year are listed for correct fitment.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	6 Cyl 3.125 Bore 2L	135	2-Stroke	1986–1991	765-7445A12	Bottom guided
Mercury	6 Cyl 3.125 Bore 2L	150	2-Stroke	1978–1991	765-7445A12	Bottom guided
Mercury	6 Cyl 3.125 Bore 2L	175	2-Stroke	1976–1981	765-7445A12	Bottom guided
Mercury	6 Cyl 3.125 Bore 2L XR2	150	2-Stroke	1986–1988	765-7445A12	Bottom guided
Mariner	Various models	135–175	2-Stroke		765-7445A12	Bottom guided

Technical Specs

Product Type: Piston Kit (Starboard .030 Oversize)
Installation: Engine block, cylinder, starboard (right) side
Fuel Type: Gasoline
Connection: Bottom guided design
Housing Material: Aluminum alloy
Housing Color: Metallic (unfinished)
Diameter: 3.155" (80.14 mm) diameter [standard bore + .030"]
Product Dimensions: 15.5" (H) x 9.5" (W) x 8.5" (L)
Fitting Type: Used with connecting rod #636
Warranty: Manufacturer standard (varies by brand)
Other Technical Attributes: .030" oversize (compared to standard 3.125")

What’s in the Box

1 x Piston Assembly - Starboard .030 Oversize (765-7445A12)
1 x Piston Ring Set (.030 Oversize) (85636A4)
1 x Wrist Pin (Included in kit)
2 x Wrist Pin Clips (Included in kit)

Installation Notes

Disconnect the battery and fuel supply as a safety precaution.
Remove the engine cover and disassemble the powerhead to access the cylinders.
Carefully remove the cylinder head and identify the starboard-side piston to be replaced.
Remove the old piston, connecting rod, and wrist pin assembly.
Inspect the cylinder bore for scoring or damage. This .030" oversize piston requires the cylinder to be professionally bored and honed to match.
Install the new piston rings onto the piston, ensuring correct orientation and gap alignment per engine service manual specifications.
Assemble the new piston, wrist pin, and clips onto the connecting rod.
Lubricate the piston, rings, and cylinder wall with 2-stroke engine oil before carefully compressing the rings and inserting the piston into the cylinder.
Reassemble the engine components in reverse order, torquing all fasteners to the manufacturer’s specifications.
After assembly, perform a compression test and follow proper engine break-in procedures.

Troubleshooting & FAQ

How do I know if I need an oversize piston?

An oversize piston is required when the engine cylinder is worn or damaged beyond the service limits for a standard-bore piston. A marine mechanic must measure the cylinder bore; if it needs to be machined (bored) to create a smooth, round surface, an oversize piston is used to match the new, larger diameter.

Is there a difference in port vs. starboard pistons?

Yes, for this engine series, port and starboard pistons are different. This kit is specifically for the starboard (right) side. Using the incorrect piston can lead to catastrophic engine failure. Always verify the correct application before installation.

What does "bottom guided" mean?

"Bottom guided" refers to the connecting rod design, where the rod is guided by the crankshaft journal. This piston is specifically designed to work with the #636 bottom-guided connecting rod used in these Mercury/Mariner engines.

What are the signs of a failing piston?

Common symptoms include loss of compression in one or more cylinders, excessive smoke, a knocking or rattling sound from the engine (piston slap), and reduced power or poor performance. A compression test is the most direct way to diagnose piston or ring issues.

OEM vs. Aftermarket?

This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9001 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

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Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.

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