SEI SE421 Lower Unit for Yamaha 76-Degree V6 | 90-421 | 2 & 4-Stroke | Not Eligible For Free Shipping

SEI SE421 — Lower Unit for Yamaha 76-Degree V6 | 2 & 4-Stroke | Part 90-421 | Not Eligible For Free Shipping

SEI SE421 (part number 90-421) is the aftermarket replacement lower unit for most Yamaha 76-degree V6 outboard engines, available in both 2-stroke and 4-stroke configurations, and in 20", 25", and 30" shaft lengths. Ships in black — can be painted to match your engine color. SEI 3-year fault-free warranty included. Not eligible for free shipping.

Important — confirm before ordering:

• Not designed for Yamaha F250 four-stroke 250 HP motors
• Fits most Yamaha 76-degree V6 applications — confirm your specific engine model before ordering

Confirm Fitment Before Ordering

Fitment depends on your Yamaha engine model, stroke type, and shaft length. Contact us with your Yamaha engine model and serial number by email, text, or live chat to confirm SE421 is the correct lower unit for your engine before ordering.

Part Specifications

• SEI Part Number: 90-421
• Model: SE421
• Type: Aftermarket replacement lower unit
• Application: Most Yamaha 76-degree V6 outboard engines
• Stroke: 2-stroke (3.1L & 3.2L) and 4-stroke variants available
• Shaft Lengths Available: 20", 25", 30"
• Paint: Black (can be painted to match)
• Does Not Fit: Yamaha F250 four-stroke 250 HP
• Warranty: SEI 3-year fault-free warranty
• Shipping: Not eligible for free shipping
• Vendor: Sterndrive Engineering (SEI)
• Related Items: Anode Kit for SE421 | SEI 90-421-05K

Variant Selection Guide

• 3.1L & 3.2L / 2-Stroke / 20" Shaft — 2-stroke Yamaha V6 3.1L or 3.2L, standard shaft
• 3.1L & 3.2L / 2-Stroke / 25" Shaft — 2-stroke Yamaha V6 3.1L or 3.2L, long shaft
• 3.1L & 3.2L / 2-Stroke / 30" Shaft — 2-stroke Yamaha V6 3.1L or 3.2L, extra-long shaft
• 4-Stroke / 25" Shaft — 4-stroke Yamaha V6, long shaft
• 4-Stroke / 30" Shaft — 4-stroke Yamaha V6, extra-long shaft

Additional Information

• Vendor: Sterndrive Engineering (SEI)
• Part Number: 90-421
• Application: Most Yamaha 76-degree V6 — 2 & 4-stroke
• Shaft Options: 20", 25", 30"
• Does Not Fit: Yamaha F250 four-stroke 250 HP
• Warranty: SEI 3-year fault-free
• Shipping: Not eligible for free shipping

Installation Notes

Install following the applicable Yamaha outboard service manual for your engine model and year. Install a fresh anode kit at the same time as the lower unit — the SEI 90-421-05K Anode Kit is the correct companion for the SE421. The unit ships in black — apply touch-up paint to match your engine color as needed after installation.

SEI Warranty

All SEI products are warranted against defects in material and workmanship for either one (1) year or three (3) years from the date of actual purchase, as applicable. Proof of purchase is required to enforce warranty coverage. These warranties are the sole and exclusive warranty provided by SEI; all other warranties and potential damages are limited and excluded. Warranty commences on the shipment date from SEI or the date of purchase from an authorized SEI dealer. Yamaha is a trademark of Yamaha Motor Co., Ltd. Sterndrive Engineering, Inc. (SEI) has no affiliation with Yamaha Motor Co., Ltd. As with any aftermarket product, OEM warranties will not apply to SEI products or to products damaged due to the operation of an SEI product.

Frequently Asked Questions

Q: How do I confirm SE421 fits my Yamaha outboard?
A: The SE421 fits most Yamaha 76-degree V6 engines in 2-stroke and 4-stroke configurations. It does not fit the Yamaha F250 four-stroke 250 HP. Locate your engine model designation and serial number on the engine ID tag and contact us by email, text, or live chat — we’ll confirm the correct lower unit and shaft length before you order.

Q: How do I know which shaft length to select?
A: Shaft length is determined by your boat’s transom height: 20" (short shaft) fits a 15" transom, 25" (long shaft) fits a 20" transom, and 30" (extra-long shaft) fits a 25" transom. Confirm your transom height before selecting. If you are unsure, contact us with your boat make, model, and year and we’ll help identify the correct shaft length.

Q: Why does the SE421 not fit the Yamaha F250?
A: The Yamaha F250 four-stroke 250 HP uses a different lower unit case design and internal components that are not compatible with the SE421. Using an incompatible lower unit risks seal failure, gear damage, and water intrusion. Do not order the SE421 for an F250 application.

Q: Do I need an anode kit with the SE421?
A: Yes — install a fresh anode kit whenever replacing a lower unit. The SEI 90-421-05K Anode Kit is the correct companion for the SE421. Installing new anodes at the same time ensures full corrosion protection from day one.

Q: The SE421 ships in black — how do I match my engine color?
A: The SE421 is manufactured in black and can be painted to match your Yamaha engine color after installation. Use a marine-grade topside paint rated for submerged use on the lower unit surfaces. Yamaha touch-up paint is available separately — match your engine’s color code for best results.