New Aftermarket Yamaha 75-100HP Outboard Lower Unit 90-404 2-4-Stroke

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

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.

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.