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New Yamaha 75–100HP 2-/4-Stroke (1984–2017) Replacement Lower Unit – Aftermarket | Standard Rotation | Fits 20″ & 25″ Shafts | 2.0 & 2.3 Gear Ratios | 3-Year Warranty

Q: What Yamaha outboard models are compatible with this 75–100HP replacement lower unit?

This lower unit fits Yamaha 75HP to 100HP 2-stroke and 4-stroke outboards from 1984–2017, including F75, F80, F90, F100, 90TLR, and E75 models. It supports 20″ and 25″ shaft lengths and standard rotation. Excludes 2016+ Yamaha 4-stroke models ending in “B”.

Q: How do I choose the correct gear ratio and shaft length for my Yamaha 75–100HP lower unit?

Yamaha 75–90HP models typically use a 2.0:1 gear ratio, while select 100HP outboards use a 2.3:1 ratio. Shaft length must match your existing motor—20″ for standard (long) and 25″ for extra-long versions. Check your engine’s model tag or Yamaha service manual for the correct configuration.

Q: Is this SEI Marine Products lower unit an OEM or aftermarket assembly?

It’s an aftermarket lower unit built by SEI Marine Products, offering OEM-grade performance and quality. Each unit is 100% new, pressure-tested, and pre-assembled with gears and a water pump, providing a cost-effective alternative to OEM replacements.

Q: Does the Yamaha 75–100HP lower unit come pre-filled with oil or require setup before installation?

The unit ships without oil and must be filled with marine gear lube before installation. All internal components are pre-installed, so setup only requires filling gear oil, sealing plug washers, and ensuring alignment during assembly.

Q: What type of warranty coverage does this Yamaha replacement lower unit include?

This Yamaha 75–100HP lower unit includes a 3-Year Fault-Free Warranty from SEI Marine Products, covering manufacturing and performance defects under normal recreational use. It guarantees reliable long-term performance similar to OEM standards.

Original price was: $1,330.89.Current price is: $1,199.00.

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SKU: ST-90-404 Categories: Standard, Yamaha Lower Unit

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Description
Additional information

Description

Brand New Yamaha 75–100HP Replacement Lower Unit (2/4-Stroke | 1984–2017)

Enhance the performance of your Yamaha 75–100HP outboard with this high-quality aftermarket replacement lower unit from SEI Marine Products. Engineered for precision, durability, and a perfect OEM fit — this unit is the ideal choice for both 2-stroke and 4-stroke Yamaha outboards manufactured between 1984 and 2017.

Key Features

Perfect Fit for Yamaha 75–100HP Outboards: Designed specifically to replace lower units on Yamaha F75, F80, F90, F100, and other compatible models.
Wide Model Compatibility: Works seamlessly with Yamaha 2-stroke and 4-stroke engines (1984–2017) for versatile installation.
Multiple Gear Ratios & Shaft Options: Available in 2.0:1 and 2.3:1 gear ratios and fits 20″ and 25″ shafts, ensuring optimal performance for your specific Yamaha 75–100HP motor.
Durable OEM-Like Finish: Finished in black paint, ready to install — can also be repainted to match original Yamaha color for a custom OEM look.
Reliable Performance Guarantee: Comes with a 3-Year Fault-Free Warranty, backed by SEI’s trusted quality assurance.

Specifications

Feature	Details
Horsepower Range	75HP to 100HP
Stroke Type	2-Stroke & 4-Stroke
Rotation	Standard (Right-Hand)
Gear Ratios	2.0:1 and 2.3:1 (model dependent)
Shaft Lengths	20″ & 25″
Paint Color	Black (Repaintable to match OEM finish)
Fits Models	F75, F80, F90, F100, 90TLR, E75, and others (see detailed list below)
Exclusions	Not compatible with 2016+ Yamaha 4-stroke models ending in “B”
Warranty	3-Year Fault-Free Warranty
Brand	SEI Marine Products

Why Choose This Yamaha 75–100HP Lower Unit?

This SEI replacement unit is a complete, brand-new assembly — not remanufactured — offering OEM-level reliability at a fraction of the cost. Each lower unit is factory pressure-tested, ready to install, and includes pre-assembled internal gears and water pump components for effortless replacement.

Designed for Yamaha 75–100HP outboards
100% brand new aftermarket replacement
Backed by SEI’s 3-year warranty

Frequently Asked Questions – Yamaha 75–100HP Replacement Lower Unit

1. What Yamaha outboard models are compatible with this 75–100HP replacement lower unit?

This lower unit is designed for Yamaha 75HP to 100HP outboards manufactured between 1984 and 2017, including 2-stroke and 4-stroke engines. Compatible models include F75, F80, F90, F100, 90TLR, and E75. It fits standard rotation outboards and supports both 20″ and 25″ shaft lengths. Please note that it is not compatible with 2016+ 4-stroke Yamaha models ending in “B”.

2. How do I choose the correct gear ratio and shaft length for my Yamaha 75–100HP lower unit?

The correct gear ratio depends on your existing Yamaha outboard’s model and propeller setup. Most 75–90HP models use a 2.0:1 ratio, while select 100HP units use a 2.3:1 ratio. Shaft length should match your current unit — 20″ for standard (long shaft) and 25″ for extra-long shaft configurations. Refer to your engine’s model plate or Yamaha service manual for precise specifications before purchase.

3. Is this SEI Marine Products lower unit an OEM or aftermarket assembly?

This is a brand-new aftermarket replacement lower unit manufactured by SEI Marine Products. It offers OEM-level performance and reliability but at a much lower cost. Each unit is 100% new (not rebuilt or remanufactured), factory pressure-tested, and pre-assembled with quality internal gears and water pump components, making installation direct and hassle-free.

4. Does the Yamaha 75–100HP lower unit come pre-filled with oil or require setup before installation?

The unit ships dry and should be filled with high-quality marine gear lube before installation. Always add the manufacturer-recommended gear oil and replace the drain/vent plugs with new seals. It comes ready to install with the water pump and gears pre-assembled, so setup is minimal aside from lubrication and alignment checks.

5. What type of warranty coverage does this Yamaha replacement lower unit include?

SEI Marine Products provides a 3-Year Fault-Free Warranty on all new Yamaha 75–100HP replacement lower units. This warranty covers any manufacturing or material defects and even includes replacement in the event of mechanical failure under normal recreational use. It’s one of the strongest aftermarket warranties available for Yamaha outboard lower units.

Looking for other models or gear ratios?

Browse all Yamaha Lower Units here — find compatible options for 40HP, 60HP, 115HP, 150HP, and more Yamaha outboards.

Additional information

Weight	85 lbs
Dimensions	23 × 13 × 26 in
Ratio/Shaft/Motor	2.0 Ratio/20" 18 spline/2-Stroke/RH Rotation , 2.3 Ratio/20" 18 Spline/4-Stroke , 2.3 Ratio/25" 18 Spline/4-Stroke