New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

$1,645.00
$1,899.00
$1,645.00
(-13%)

MPN : 90-423

SKU: SE90-423-A25F
Availability: Out Of Stock
Please hurry! Only 0 left in stock
Liter/Shaft/Motor: 4.2L / 25" / 4 Stroke
Subtotal: $1,645.00
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

$1,899.00 $1,645.00
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423
New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

New Aftermarket Yamaha 250-350HP 4-Stroke Outboard Lower Unit 90-423

$1,899.00 $1,645.00
Liter/Shaft/Motor: 4.2L / 25" / 4 Stroke
Description
Description
x

Upgrade your Yamaha 250-350HP outboard with this SEI 90-423 Aftermarket Lower Unit, engineered as a direct-fit replacement for Yamaha F250-F350 4-Stroke engines with 4.2L displacement.
This high-quality gearcase delivers OEM-level performance and reliability at a fraction of the cost — no modifications required.

Built for durability and long service life, the unit features a water-cooled bearing carrier, dual and side water intakes, and a standard (right-hand) rotation design for optimal cooling and smooth operation in all water conditions.
Each unit comes pre-painted in black and can be easily repainted to match your outboard.

Available in 25″ and 30″ shaft lengths, this lower unit is ideal for restoring top-end performance and maintaining reliability for both freshwater and saltwater use.

Specifications

Feature Details
Compatible Engines Yamaha 4-Stroke F250-F350 (4.2L)
Horsepower 250-350 HP
Rotation Standard (Right-Hand)
Gear Ratio 2.00 : 1
Shaft Lengths 25″, 30″
Water Intake Dual & Side (Replaces both versions)
Bearing Carrier Water-cooled
Finish Black (Repaintable to match engine)
Condition New – Aftermarket
Warranty 3-Year Fault-Free Warranty
Part Numbers 90-423-A25F (25″), 90-423-A30F (30″)

Why Choose the Yamaha 250-350HP 4-Stroke Lower Unit (SEI 90-423)?

  • Direct OEM Replacement: Designed to precisely fit Yamaha F250-F350 4-Stroke outboards with 4.2L displacement — no modifications required.
  • High-Performance Engineering: Built to deliver smooth, quiet, and reliable operation comparable to Yamaha factory units.
  • Dual & Side Water Intakes: Ensures superior cooling performance, even in shallow or high-temperature waters.
  • Water-Cooled Bearing Carrier: Extends the life of internal components under heavy-duty marine use.
  • Fully Assembled & Ready to Install: Arrives pre-filled with premium gear lube for quick setup.
  • Repaintable Black Finish: Can be customized to match your outboard's original color.
  • 3-Year Warranty: Comprehensive coverage for peace of mind and long-term reliability.

Frequently Asked Questions – Yamaha 250-350HP 4-Stroke Replacement Lower Unit

1. Which Yamaha outboard models are compatible with the SEI 90-423 replacement lower unit?

The SEI 90-423 replacement lower unit is designed to fit Yamaha F250-F350 4-Stroke outboards with 4.2L displacement. It supports 25″ and 30″ shaft lengths, offering a direct-fit, no-modification solution for these engine types.

2. What is the gear ratio for this lower unit and why is it important?

This replacement lower unit features a 2.00:1 gear ratio, which matches the OEM specification for Yamaha F250-F350 models. Matching the gear ratio is critical to maintaining proper propeller performance, engine efficiency, and preventing premature wear.

3. What features does this SEI 90-423 lower unit offer for durability and performance?

The unit includes a water-cooled bearing carrier to extend component life, dual and side water intakes for superior cooling efficiency, and is constructed with OEM-level precision for smooth, quiet operation. Its black paintable finish allows easy customization to match your outboard.

4. Is this unit ready to install out of the box, or does it require additional preparation?

The SEI 90-423 arrives fully assembled and pre-filled with premium marine gear lube, making installation quick and hassle-free. It requires no modifications or additional setup beyond aligning shaft length and gear ratio compatibility with your existing engine.

5. What warranty protection comes with the SEI 90-423 Yamaha 250-350HP lower unit?

This aftermarket lower unit is backed by a 3-year fault-free warranty, covering defects in materials and workmanship as well as mechanical failures under normal recreational use, providing confidence and peace of mind for marine enthusiasts.

Looking for other models or gear ratios?
Browse all Yamaha Lower Units here — find compatible options for various Yamaha outboards.

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