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Required for fitment review to confirm compatibility.
The SEI Mercury SE205 lower unit is a new aftermarket replacement gearcase for selected Mercury and Mariner 3- and 4-cylinder outboards.
The SE205 family includes separate configurations for 2-stroke carbureted, DFI and early four-stroke applications, with 2.07:1 or 2.33:1 ratios and 20-inch or 25-inch shaft options.
This product does not fit every engine between 70 and 125 HP. Before ordering, verify:
If any detail is uncertain, contact Mariners Warehouse before ordering.
Before your SEI Mercury SE205 lower unit is released for shipment, the submitted engine information is reviewed against the selected gear ratio, engine type, shaft length and SEI part number.
To help prevent a wrong order, please provide your complete Mercury engine serial number and confirm whether your application is:
If the submitted information appears inconsistent with the selected Mercury replacement lower unit, Mariners Warehouse may contact you for clarification before fulfillment.
When the shipment arrives, inspect the package and lower unit for visible shipping damage. Confirm the SKU, shaft configuration and product condition before installation.
Do not install, fill or operate the lower unit if the received SKU or configuration differs from your order. Contact Mariners Warehouse before proceeding.
| Specification | Mercury SE205 Outboard Lower Unit | 70–125 HP | SE90-205 |
|---|---|
| Manufacturer | SEI Marine |
| Product family | SE205 |
| Parent part number | 90-205 |
| Product | New aftermarket replacement lower unit |
| Compatible brand | Mercury and Mariner |
| Primary application | Selected 3- and 4-cylinder outboards |
| 2-stroke coverage | Most selected 70–125 HP applications from 1987 and newer |
| DFI coverage | Selected DFI applications; engine serial verification required |
| 4-stroke coverage | Selected 75–115 HP applications below serial 0T801000 |
| Gear ratios | 2.07:1 or 2.33:1 |
| Earlier 2.31 ratio | SEI specifies 2.33:1 as the current replacement |
| Shaft options | 20-inch L and 25-inch XL |
| Bullet diameter | 4-1/4 inches only |
| 22.5-inch LL shaft | Not offered |
| Plastic water-pump housing | Not compatible |
| 3-jaw application | Midsection shift-shaft update may be required |
| Required information | Complete engine serial number |
| Warranty | 3-year SEI fault-free manufacturer warranty; terms apply |
Selecting a Mercury 70–125 HP lower unit requires more than matching the horsepower printed on the cowling. The SEI SE205 family contains different gearcases for carbureted two-stroke, DFI and early four-stroke engines.
Begin with the complete engine serial number. Next, confirm whether the existing gear ratio is 2.07:1 or 2.31/2.33:1. SEI identifies 2.33:1 as the current replacement for applicable 2.31-ratio units.
You must also choose the correct 20-inch or 25-inch shaft and confirm a 4¼-inch bullet diameter. A 22.5-inch LL shaft is not offered.
If the original lower unit uses a 3-jaw clutch, applications of 75 HP and above require evaluation of the midsection shift shaft and may need the separate shift-shaft kit.
The safest ordering method is to match the complete serial number, engine type, ratio, shaft length and selected SEI SKU before checkout.
Use CTRL + F or search below to find your engine model
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The product information made it easy to understand which SEI SE205 lower unit configuration was needed for a Mercury 90 HP two-stroke application. The 2.07:1 ratio, 20-inch shaft, and serial-number requirements were clearly explained. Buyers should verify every fitment detail instead of ordering by horsepower alone.
I appreciated that the Mercury DFI lower unit options were separated from the carbureted and four-stroke configurations. The 25-inch shaft, 2.07:1 ratio, and engine serial-number guidance made the selection process much clearer. Confirming the exact DFI application before ordering is important.
The SEI Mercury SE205 lower unit page clearly explained the early four-stroke serial-number limitation and the required 4-1/4-inch bullet diameter. The 20-inch, 2.07:1 configuration was easy to identify. The fitment warnings were more helpful than a generic horsepower list.
The most important detail for this Mercury replacement lower unit was matching the 2.33:1 gear ratio. The page made it easy to compare the engine type and 20-inch shaft configuration. I would recommend checking the original ratio and complete engine serial number before placing an order.
The BigFoot-specific information helped distinguish this configuration from a standard Mercury lower unit. The 40-60 HP four-stroke application, 2.33:1 ratio, and 20-inch shaft were clearly identified. The serial-number verification requirement is an important step before installation.
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