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Required for fitment review to confirm compatibility.
The SEI SE218 Mercury 3.0L counter-rotation lower unit is a new aftermarket replacement gearcase for selected Mercury 3.0L V6 outboards with serial number 1B504989 and below. It is designed specifically for applications requiring counter/left-hand rotation and is available in 1.75:1 and 1.62:1 gear-ratio configurations.
Regular SE218 configurations use a 13-spline one-piece driveshaft in the listed 25-inch or 30-inch length. Stub/Alpha configurations are available for applicable original 8-spline arrangements, but they require a compatible OE upper driveshaft purchased separately.
Fitment cannot be determined by displacement, horsepower, model year or engine position alone. Before ordering, verify the complete engine serial number, required rotation, existing gear ratio, driveshaft length and spline configuration.
The SE218 does not replace a Torque Master, Torque Master 2 or Sport Master gearcase. If the application requires standard/right-hand rotation, review the SEI SE216 instead.
The SE218 is intended for selected Mercury 3.0L V6 outboards requiring a counter/left-hand rotation lower unit.
Confirmed family-level requirements:
Do not use an approximate model year or horsepower range as the sole fitment method. Mercury used different gearcase families within similar horsepower and production ranges. Submit the complete engine serial number and existing lower-unit information before ordering.
Before shipment, the selected SE218 configuration is reviewed using the engine information submitted with the order. Provide the complete engine serial number and select the correct gear ratio, driveshaft configuration and counter/left-hand rotation.
The review should include:
If the application originally uses an 8-spline driveshaft, a compatible OE upper driveshaft is required with the Stub/Alpha unit. For serial numbers below 0G438000, confirm whether the existing water-tube coupling can be reused or whether SEI part 96-116-12K is required.
When the shipment arrives:
Do not install, fill, modify, paint or operate the gearcase if the configuration appears incorrect or transit damage is visible. Contact Mariners Warehouse before installation.
Before operation, the installer must verify the required installation components, lubricant specification and fill level according to the applicable SEI instructions. Fitment review does not replace final verification and installation by a qualified marine technician.
| Specification | Mercury SE218 Outboard Lower Unit |3.0L Counter-Rotation| SE90-218 |
|---|---|
| Product | New aftermarket replacement gearcase |
| Manufacturer | SEI Marine |
| Product family | SE218 |
| Engine family | Selected Mercury 3.0L V6 outboards |
| Serial range | 1B504989 and below |
| Rotation | Counter/left-hand only |
| Gear ratios | 1.75:1 and 1.62:1 |
| Gear teeth | 1.75:1: 12/21; 1.62:1: 17/28 or 13/21 |
| One-piece driveshafts | 13-spline configurations |
| Shaft options | 25-inch XL and selected 30-inch XXL configurations |
| Stub/Alpha options | Available for applicable original 8-spline arrangements; compatible OE upper driveshaft required |
| Water-tube coupling | Included for serial number 0G438000 and higher |
| Early coupling requirement | Below 0G438000, reuse the existing coupling or use SEI 96-116-12K |
| Exclusions | Does not replace Torque Master, Torque Master 2 or Sport Master |
| Current SKUs | 90-218-A25, 90-218-A30, 90-218-AS, 90-218-B25 and 90-218-BS |
Choosing the correct Mercury 3.0L counter-rotation lower unit requires more than identifying the engine’s displacement or its position on the transom. Begin with the complete engine serial number. The SEI SE218 is intended for selected Mercury 3.0L V6 applications with serial number 1B504989 and below.
Next, verify that the existing application actually requires counter/left-hand rotation. Counter rotation is commonly used in multi-engine installations, but engine position alone does not confirm the required gearcase.
The existing gear ratio must also be matched. SE218 configurations are available in 1.75:1 and 1.62:1 ratios. Installing the wrong ratio can create a mismatch within a multi-engine application.
Finally, determine whether the application uses a 13-spline one-piece driveshaft or an original 8-spline arrangement. An 8-spline application requires the appropriate SE218 Stub/Alpha configuration plus a compatible OE upper driveshaft.
Do not select the SE218 for Torque Master, Torque Master 2, Sport Master or standard-rotation applications. Submit the engine serial number whenever any fitment detail is uncertain.
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The product information made it easy to understand which SEI SE218 lower unit configuration was needed for a 2020 Mercury 250 HP V8 FourStroke application. The 1.75:1 gear ratio, 20-inch shaft length, and serial-number verification steps were clearly explained. Buyers should double-check their exact gear ratio before ordering.
I appreciated that the SEI SE218 lower unit options clearly highlighted compatibility for higher horsepower Mercury applications. The 25-inch XL shaft, 1.85:1 gear ratio, and engine serial-number guidance made selecting the right model effortless. Confirming rotation type before ordering is essential.
The SEI Mercury SE218 lower unit page provided clear details regarding mounting stud alignments and drive shaft length options for my 2018 Mercury 225 HP. The standard rotation 20-inch model was easy to locate, and the packaging arrived well-protected with dense foam packaging.
Matching the 1.75:1 gear ratio was the most critical factor for this replacement lower unit. Mariners Warehouse made it straightforward to cross-reference my engine year and horsepower. Highly recommend verifying your full engine serial number prior to placing the order.
The SE218 guidance helped me select the correct counter-rotation unit for my offshore setup. The unit arrived safely boxed without any shipping damage. It bolted right up without modification, and after finishing the initial 10-hour break-in period, performance has been rock solid.
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