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Required for fitment review to confirm compatibility.
The SEI SE221 Mercury lower unit is a new aftermarket replacement gearcase for selected Mercury outboards using the 4.8-inch torpedo-style housing and a 15-spline propeller shaft. Unlike a single-application gearcase, the SE221 family supports three distinct engine architectures, so the correct configuration must be selected using the engine serial number, engine family, original gear ratio and driveshaft length.
For selected 3.0L V6 two-stroke Mercury outboards, the SE221 uses a 13-spline driveshaft and a 1.75:1 gear ratio. These applications are associated with Mercury’s transition from the earlier SE216-style gearcase around serial number 1B504989.
For selected 2.6L inline-six Verado four-strokes, the SE221 uses a 20-spline driveshaft and is available with 1.75:1 or 1.85:1 ratios in 20-, 25- and 30-inch configurations. For selected 1.7L inline-four supercharged Verado four-strokes, 2.08:1 configurations are available with 20- and 25-inch driveshafts.
This is a standard-rotation replacement lower unit. It does not replace the SE228 counter-rotation family, a 5.44-inch Verado gearcase, Torque Master, Torque Master 2 or Sport Master applications. Later Mercury V6 and V8 engines carrying the Verado name also use different gearcases.
Because several incompatible Mercury lower units look similar externally, do not order by horsepower or appearance alone. Submit the complete engine serial number and verify all application details before installation.
Selected 3.0L V6 two-stroke applications
Selected 2.6L inline-six Verado applications
Selected 1.7L inline-four Verado applications
Before shipment, the selected SE221 configuration should be reviewed against the engine information submitted with the order. Please provide the complete engine serial number, model number, engine architecture, existing ratio, driveshaft length, driveshaft spline count and rotation.
The review should confirm whether the order is for:
The selected configuration should also be checked for the correct 4.8-inch torpedo-style gearcase and 15-spline propeller shaft. If the supplied details indicate a 5.44-inch torpedo, 19-spline fat shaft, counter rotation, Torque Master, Torque Master 2, Sport Master or later V6/V8 Verado application, the order should be paused for clarification.
When the shipment arrives:
A pre-shipment fitment review helps identify application conflicts, but it cannot guarantee installation compatibility when engine information is incomplete or incorrect.
| Specification | Mercury SE221 Outboard Lower Unit | 3.0L V6 & Verado 4.8-Inch |SE90-221 |
|---|---|
| Product | New aftermarket replacement gearcase |
| Manufacturer | SEI Marine |
| Product family | SE221 / 90-221 |
| Rotation | Standard/right-hand only |
| Gearcase style | 4.8-inch torpedo family; approximately 4.88 inches on individual SEI listings |
| Propeller shaft | 15-spline |
| Engine family 1 | Selected Mercury 3.0L V6 two-strokes with 13-spline driveshaft |
| Engine family 2 | Selected Mercury 2.6L inline-6 Verado four-strokes with 20-spline driveshaft |
| Engine family 3 | Selected Mercury 1.7L inline-4 supercharged Verado four-strokes with 20-spline driveshaft |
| V6 serial transition | Transition occurs around serial 1B504989; verify the exact serial before ordering |
| 2.6L I6 horsepower | Selected 200–300 HP applications |
| 1.7L I4 horsepower | Selected 135–200 HP supercharged applications |
| Gear ratios | 1.75:1, 1.85:1 and 2.08:1 depending on engine family |
| Shaft options | 20-inch, 25-inch and selected 30-inch configurations |
| Major exclusions | 5.44-inch torpedo, Torque Master, Torque Master 2, Sport Master and later V6/V8 Verado |
| Not included | Installation kit and anode kit |
Start with the engine architecture, not horsepower. The SE221 family covers a 3.0L V6 two-stroke, a 2.6L inline-six Verado four-stroke and a 1.7L inline-four Verado four-stroke. These applications require different driveshaft spline counts and gear ratios even though some gearcases may look similar externally.
Next, confirm the 4.8-inch torpedo-style housing. The SE221 is not a replacement for the larger 5.44-inch Verado gearcase. You should also confirm a 15-spline propeller shaft. A 19-spline fat shaft may indicate a Torque Master, Torque Master 2 or Sport Master application that is not covered by the SE221.
After confirming the engine family, match the existing gear ratio and driveshaft length. Selected 3.0L V6 applications use a 1.75:1 ratio. The 2.6L inline-six Verado applications may use 1.75:1 or 1.85:1, while the supported 1.7L inline-four Verado applications use 2.08:1.
Finally, provide the complete engine serial number. This is especially important for 3.0L V6 engines near the SE216-to-SE221 transition around serial number 1B504989. Do not order solely by horsepower, year or external appearance.
Use CTRL + F or search below to find your engine model
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The product details made it simple to confirm that the SEI SE221 lower unit was the right replacement for my 2020 Mercury 250 HP V8. The 1.75:1 gear ratio, 20-inch shaft length, and serial-number verification instructions were very clear. Always double-check your gear ratio before placing an order.
I appreciated how clearly the SE221 lower unit options specified compatibility with late-model Mercury Verado high-horsepower outboards. The 25-inch XL shaft, 1.85:1 gear ratio, and serial-number guide made selection straightforward. Confirming rotation type in advance is a must.
The SEI Mercury SE221 lower unit page provided detailed guidance on drive shaft lengths and mounting stud alignment for my 2018 Mercury 225 HP. The 20-inch standard rotation model was easy to identify, and the unit arrived in sturdy, well-cushioned packaging.
Matching the 1.75:1 gear ratio was the most critical requirement for my replacement lower unit. Mariners Warehouse made cross-referencing my motor's year and horsepower very easy. I recommend checking your complete serial number before buying.
The SE221-specific documentation made it easy to pick the correct counter-rotation unit for my twin engine setup. The shipment arrived safely boxed without damage. Installation went smoothly, and after completing the mandatory 10-hour break-in period, performance has been excellent.
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