Pinion and Forward Gear Assembly – Fits Mercury/Mariner 3.0L 200–300 HP (Replaces 43-859321A3)

$490.93 $785.00

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Product Details

This aftermarket Pinion and Forward Gear Assembly is engineered for Mercury and Mariner 3.0L V6 outboards, approximately from 1998 to 2008. It is designed for standard rotation lower units requiring a 1.75:1 gear ratio, fitting a wide range of 200–300 HP models.

It serves as a direct replacement for Mercury part numbers 43-859321A3 and 859321A3, as well as aftermarket equivalents like WSM 444-50 and the forward gear component of SEI Marine Products kit 91-216-03AKS.

Installs in the lower unit gearcase.
Constructed from durable, marine-grade metal alloy.
Features a non-ratcheting gear design.

Part Number Matrix

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

Brand	Part Number	Notes
Mercury	43-859321A3	OEM
Mercury	859321A3	OEM
WSM	444-50	Aftermarket
SEI Marine Products	91-216-03AKS	Forward gear component of set

Engine & Model Compatibility

Confirm your engine model/year in the table below.

For Mercury/Mariner outboards, check your model, horsepower, and serial number (S/N) range to ensure an exact match.

For Mercury Racing models, verify the specific engine series and serial number.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury	200 (3.0L DFI)(DTS)	200	2-Stroke	2003–2008	43-859321A3	S/N 0T980000-1B504988
Mercury	200 (3.0L EFI)	200	2-Stroke	1999–2008	43-859321A3	S/N 0T750164-1B504988
Mercury	200 (DFI)	200	2-Stroke	1998–2008	43-859321A3	S/N 0G960500-1B504988
Mercury	225 (3.0L CARB)	225	2-Stroke	1998–2001	43-859321A3	S/N 0G760300-0T408999
Mercury	225 (3.0L EFI)	225	2-Stroke	1999–2008	43-859321A3	S/N 0T409000-1B504988
Mercury	225 (3.0L EFI) MARATHON	225	2-Stroke	2001–2008	43-859321A3	S/N 0T801000-1B417701
Mercury	225 (3.0L EFI) SEAPRO	225	2-Stroke	2001–2008	43-859321A3	S/N 0T801000-1B417701
Mercury	225 (3.0L-DFI)(DTS)	225	2-Stroke	1999–2008	43-859321A3	S/N 0T599000-1B504988
Mercury	225 (DFI)	225	2-Stroke	1998–2008	43-859321A3	S/N 0G960500-1B504988
Mercury	225 MARATHON (3.0L CARB)	225	2-Stroke	1998–2001	43-859321A3	S/N 0G760300-0T408999
Mercury	225 SEAPRO (3.0L CARB)	225	2-Stroke	1998–2001	43-859321A3	S/N 0G760300-0T408999
Mercury	250 (3.0L EFI)	250	2-Stroke	1998–2008	43-859321A3	S/N 0G760300-1B504988
Mercury	3.0L EFI MARATHON	250	2-Stroke	1999–2001	43-859321A3	S/N 0T409000-0T800999
Mercury	3.0L EFI SEAPRO	250	2-Stroke	1999–2001	43-859321A3	S/N 0T409000-0T800999
Mercury	Pro Max 300 (3.0L)	300	2-Stroke	1998–2005	43-859321A3	S/N varies
Mercury	Optimax 200 (3.0L)	200	2-Stroke	2000–2007	43-859321A3
Mercury	Optimax 225 (3.0L)	225	2-Stroke	2000–2007	43-859321A3
Mercury	Optimax 250 (3.0L)	250	2-Stroke	2003–2007	43-859321A3
Mercury Racing	250XS (3.0L DFI)	250	2-Stroke	1999–2008	43-859321A3	S/N 0T651945-1E000796
Mercury Racing	300X (3.0L EFI) PRO MAX	300	2-Stroke	1998–2003	43-859321A3	S/N 0T235084-0T999587

Technical Specs

Product Type: Pinion and Forward Gear Assembly
Installation: Lower Unit/Gearcase
Gear Ratio: 1.75:1
Tooth Count: 12/21
Rotation: Standard
Housing Material: Marine-grade metal alloy
Product Dimensions: 15.5″ L x 11.5″ W x 12.0″ H
Other Technical Attributes: Non-ratcheting gear set

What’s in the Box

1 x Gear Set, Forward/Pinion (Replaces 43-859321A3)

Installation Notes

Safely secure the outboard motor and disconnect the battery.
Drain the gearcase oil completely into a suitable container.
Remove the propeller assembly and the lower unit mounting bolts.
Carefully detach the lower unit (gearcase) from the outboard's upper housing.
Disassemble the gearcase to access the driveshaft, pinion gear, and forward gear. Note the position and thickness of all shims.
Remove the old gears and inspect all bearings, seals, and the driveshaft for wear or damage. Replace as necessary.
Install the new pinion and forward gear, using the original shims as a starting point.
Measure the gear backlash according to your engine's service manual and adjust shimming for correct tolerance.
Reassemble the gearcase using new gaskets and seals.
Re-install the lower unit, propeller, and refill with the manufacturer-specified marine gear lube.

Troubleshooting & FAQ

How do I know if this gear set fits my specific engine model?

The most reliable method is to cross-reference your engine's original part number with those listed in our Part Number Matrix. Additionally, please review the Engine & Model Compatibility table and match your engine's model, horsepower, year range, and serial number. If you are unsure, consult a qualified marine mechanic or contact us with your engine details.

What are the common signs of a failing forward or pinion gear?

Symptoms of gear failure include grinding or clicking noises from the lower unit, excessive metal shavings or chunks found in the gear lube during a change, the engine revving without corresponding propeller speed (slipping), or difficulty shifting into forward gear.

Is this a ratcheting or non-ratcheting gear set?

This is a non-ratcheting gear set. It's critical to use the correct type for your specific application, as mixing them can cause severe damage.

Do I need to adjust the shimming and backlash when installing?

Yes, absolutely. Proper gear alignment is critical for performance and longevity. You must measure and set the pinion height and gear backlash using a dial indicator, following the procedures in your OEM service manual. Failure to do so will lead to rapid gear wear and catastrophic failure.

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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