Mercury Piston Assembly – Fits Mercury/Mariner 30 Jet/40/50 HP Outboards (Replaces 780-9229A7)

$137.35 $219.00

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Delivery and Shipping

Free standard shipping on all orders under 1 KG

If you have any questions, feel free to email us at info@sierramarines.com, and we’ll get back to you within 20–36 hours.

Product Details

This is an aftermarket Mercury Piston Assembly designed to fit a range of Mercury and Mariner 4-cylinder, 2-stroke outboards, including 30 Jet, 40 HP, 45 HP, and 50 HP models manufactured between 1972 and 2005. It features a crossflow design with a deflector cast into the piston top to optimize the air/fuel mixture flow, ensuring reliable performance.

This assembly directly replaces Mercury OEM part numbers 780-9229A7, 8172A6, 3193A4, 3193A2, and the superseded part 5172A6.

Complete Kit: Includes the piston, rings, wrist pin, and retaining clips for a full replacement.
Direct Fit: Engineered as a standard bore (STD) size replacement for engine block cylinders.
Crossflow Design: Features an integrated deflector for efficient fuel circulation in compatible engines.

Part Number Matrix

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

Brand	Part Number	Superseded By
Mercury	780-9229A7
Mercury	5172A6	780-9229A7
Mercury	8172A6
Mercury	3193A2
Mercury	3193A4

Engine & Model Compatibility

Confirm your engine model/year in the table below.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	30 JET	30	2-Stroke	1995–2005	780-9229A7	From Serial No. 9570790 & Up
Mercury	30 JET	30	2-Stroke	2001–2005	780-9229A7	Serial No. 0C159200 - 0G589999
Mercury	40 (4 CYL.)	40	2-Stroke	1995–2005	780-9229A7	From Serial No. 9570790 & Up
Mercury	40 (4 CYL.)	40	2-Stroke	2001–2005	780-9229A7	From Serial No. 0C159200 & Up
Mercury	400 (4 CYL.)	40	2-Stroke	1995–2005	780-9229A7	From Serial No. 9570790 & Up
Mercury	Mariner 40-50 HP	40-50	2-Stroke	1972–1997	5172A6/780-9229A7	Crossflow engines

Technical Specs

Product Type: Piston Assembly
Installation: Engine Block Cylinder
Piston Material: Aluminum Alloy
Piston Diameter: 65.15 mm (2.565 in)
Piston Height: 70 mm (2.756 in)
Wrist Pin Diameter: 18 mm (0.709 in)
Weight: 0.318 kg (0.70 lb)
Design Type: Crossflow with Deflector
Bore Size: Standard (STD)

What’s in the Box

1 x Piston Assembly (Replaces 780-9229A7)
1 x Piston Ring Set
1 x Wrist Pin
2 x Retaining Clips

Installation Notes

Disconnect the battery and drain engine fluids before beginning work.
Remove the cylinder head and other components to access the engine block.
Carefully remove the old piston, connecting rod, and associated bearings.
Thoroughly clean the cylinder walls and inspect for any scoring or out-of-spec wear.
Lightly lubricate the new piston, rings, and cylinder wall with 2-stroke engine oil.
Use a piston ring compressor tool to carefully install the new piston assembly into the cylinder.
Re-install the connecting rod, ensuring bearings are properly seated and lubricated.
Reassemble the engine components in reverse order of removal.
Use a torque wrench to tighten all fasteners to the manufacturer's specifications, found in your engine's service manual.
Refill engine fluids, reconnect the battery, and check for proper operation.

Troubleshooting & FAQ

How do I confirm this piston fits my specific engine?

The best way is to match your engine's model, year, and serial number to the information in the compatibility table. The serial number ranges are critical for ensuring an exact fit.

Is this a standard size or oversized piston?

This is a standard size (STD) piston. It is intended for use in an engine cylinder that has not been bored or honed oversize. Always measure your cylinder bore before ordering.

What is a crossflow piston design?

A crossflow piston has a raised dome or deflector on its crown. This shape helps direct the incoming air/fuel mixture upward into the cylinder, improving scavenging and overall engine efficiency, a common design in vintage 2-stroke engines.

What are the signs of a failing piston?

Common symptoms include a loss of compression in one or more cylinders, a "knocking" or "slapping" sound from the engine, excessive smoke, and decreased performance. A visual inspection may reveal scoring on the piston skirt or cylinder wall.

Does this kit include a new wrist pin bearing?

No, this assembly includes the piston, rings, wrist pin, and clips only. The connecting rod's needle bearing is a separate component and should be inspected and replaced if necessary.

Can I reuse my old piston rings?

No. You should never reuse old piston rings. This kit includes a new, precisely matched set of rings that are essential for achieving proper compression and sealing.

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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