SEI SE421 Lower Unit – Fits Yamaha 76° V6 200–250 HP (Replaces 65J-45300-21-8D)
SEI SE421 Lower Unit – Fits Yamaha 76° V6 200–250 HP (Replaces 65J-45300-21-8D) is backordered and will ship as soon as it is back in stock.
Worry-free returns
Worry-free returns
Shop with confidence. If you’re not completely satisfied, eligible items may be returned within the return window, provided they are unused and in original packaging.
If the return is due to our error or a defective item, we’ll make it right.
Please contact us before returning any item.
Delivery and Shipping
Delivery and Shipping
Orders are typically processed within 1–2 business days.
U.S. orders usually arrive within 6–15 days, depending on stock location and destination.
International delivery times may vary due to customs processing.
Tracking information will be provided once your order ships.
SEI SE421 Lower Unit – Fits Yamaha 76° V6 200–250 HP (Replaces 65J-...
SEI SE421 Lower Unit – Fits Yamaha 76° V6 200–250 HP (Replaces 65J-45300-21-8D) is backordered and will ship as soon as it is back in stock.
Product Details
This is a complete aftermarket SEI SE421 Lower Unit, engineered as a direct replacement for Yamaha 76° V6 2-Stroke engines, including 3.1L and 3.2L models from 1990 to 2005. It features a standard right-hand rotation, a 1.81:1 gear ratio, and fits models with a 25" shaft length.
This assembly serves as a direct interchange for Yamaha OEM part numbers 65J-45300-21-8D and 60V-45300-20-8D, ensuring a precise and reliable fit.
- Provides a direct bolt-on installation with no modifications required.
- Constructed from marine-grade aluminum and finished with a paintable black primer.
- Backed by an industry-leading 3-Year No-Fault Warranty for peace of mind.
Part Number Matrix
Use this matrix to confirm equivalent references across OEM and aftermarket.
| Brand | Part Number | Notes |
|---|---|---|
| SEI | 90-421-A25 | This Complete Lower Unit Assembly |
| Yamaha | 65J-45300-21-8D | OEM Direct Replacement |
| Yamaha | 60V-45300-20-8D | OEM Direct Replacement |
Engine & Model Compatibility
Confirm your engine model/year in the table below.
This lower unit is compatible with the following Yamaha 2-stroke models.
| Model | Rated HP | Stroke | Years | OEM Part No. |
|---|---|---|---|---|
| Z250TXRD | 250 | 2-Stroke | 2005 | 65J-45300-21-8D |
| Z250TXRC | 250 | 2-Stroke | 2004 | 65J-45300-21-8D |
| SX225TXRB | 225 | 2-Stroke | 2003 | 65J-45300-21-8D |
| SX250TXRB | 250 | 2-Stroke | 2003 | 65J-45300-21-8D |
| Z250TXRB | 250 | 2-Stroke | 2003 | 65J-45300-21-8D |
| SX225TXRA | 225 | 2-Stroke | 2002 | 65J-45300-21-8D |
| SX250TXRA | 250 | 2-Stroke | 2002 | 65J-45300-21-8D |
| SX225TXRZ | 225 | 2-Stroke | 2001 | 65J-45300-21-8D |
| SX250TXRZ | 250 | 2-Stroke | 2001 | 65J-45300-21-8D |
| SX225TXRY | 225 | 2-Stroke | 2000 | 65J-45300-21-8D |
| SX250TXRY | 250 | 2-Stroke | 2000 | 65J-45300-21-8D |
| S225TXRX | 225 | 2-Stroke | 1999 | 65J-45300-21-8D |
| S250TXRX | 250 | 2-Stroke | 1999 | 65J-45300-21-8D |
| V225TXRX | 225 | 2-Stroke | 1999 | 65J-45300-21-8D |
| S225TXRW | 225 | 2-Stroke | 1998 | 65J-45300-21-8D |
| S250TXRW | 250 | 2-Stroke | 1998 | 65J-45300-21-8D |
| S225TXRV | 225 | 2-Stroke | 1997 | 65J-45300-21-8D |
| S250TXRV | 250 | 2-Stroke | 1997 | 65J-45300-21-8D |
| S225TXRU | 225 | 2-Stroke | 1996 | 65J-45300-21-8D |
| S250TXRU | 250 | 2-Stroke | 1996 | 65J-45300-21-8D |
| 225TXRT | 225 | 2-Stroke | 1995 | 65J-45300-21-8D |
| 250TXRT | 250 | 2-Stroke | 1995 | 65J-45300-21-8D |
| 225TXRS | 225 | 2-Stroke | 1994 | 65J-45300-21-8D |
| 250TXRS | 250 | 2-Stroke | 1994 | 65J-45300-21-8D |
| 250TXRR | 250 | 2-Stroke | 1993 | 65J-45300-21-8D |
| 250TXRQ | 250 | 2-Stroke | 1992 | 65J-45300-21-8D |
| 250TXRP | 250 | 2-Stroke | 1991 | 65J-45300-21-8D |
| 250ETXD | 250 | 2-Stroke | 1990 | 65J-45300-21-8D |
Technical Specs
- Product Type
- Complete Lower Unit / Gearcase Assembly
- Installation
- Direct bolt-on replacement
- Gear Ratio
- 1.81:1
- Prop Rotation
- Standard Right-Hand
- Drive Shaft Length
- 25" (635 mm)
- Housing Material
- Marine-grade aluminum
- Housing Color
- Black primer (paintable)
- Compatibility
- Yamaha 76° V6 2-Stroke 3.1L & 3.2L engines
- Cooling System
- Standard side water intakes
- Prop Shaft
- 15-spline
- Max RPM
- 5,500–6,000 (estimated)
- Product Dimensions
- 129.0 cm x 22.0 cm x 76.0 cm
- Box Dimensions
- 129.0 cm x 22.0 cm x 76.0 cm
- Warranty
- 3-Year No-Fault Warranty
What’s in the Box
- 1 x Complete Lower Unit Assembly (90-421-A25)
Installation Notes
- Ensure the engine is off and the battery is disconnected for safety. Drain the old gear lube completely.
- Carefully unbolt and remove the old lower unit from the engine housing.
- It is critical to install a new water pump kit (sold separately) onto the new lower unit.
- Install a new anode kit (sold separately) to protect against corrosion.
- Align the driveshaft and shift shaft, then bolt the new lower unit securely to the engine.
- Fill the gearcase with fresh, high-quality marine gear lube according to manufacturer specifications before starting the engine.
Troubleshooting & FAQ
Will this fit my Yamaha 250 HP 4-stroke?
No, this unit is specifically designed for Yamaha 76° V6 2-Stroke 3.1L & 3.2L engines from 1990–2005. It is not compatible with 4-stroke models.
Does this come pre-filled with gear lube?
No, the lower unit is shipped dry to prevent leaks during transit. You must fill it with the correct type and amount of gear lube before operation. We recommend the 88-102-01K Gear Lube and Pump Kit.
What are the signs my lower unit is failing?
Common signs include excessive vibration, grinding noises from the gearcase, difficulty or inability to shift gears, a milky or metallic appearance in the gear lube, or visible damage to the housing, skeg, or prop shaft.
What maintenance is required for this lower unit?
We recommend changing the gear lube and inspecting the water pump impeller annually or every 100 hours of use. You should also regularly inspect the anode and replace it when it is 50% corroded to protect the unit.
Is the water pump included with this assembly?
A water pump kit is not included. For reliability and to protect your warranty, we strongly recommend installing a new Water Pump Kit with Housing (part #96-416-01AK) when you install the new lower unit.
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 3-Year No-Fault 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.
Payment & Security
Payment methods
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
