Evinrude & Johnson Lower Unit Fitment Guide: SE304 to SE318
Evinrude & Johnson Lower Unit Fitment Guide: SE304 to SE318
A model-by-model comparison to help you match the correct Evinrude or Johnson lower unit to your outboard before you buy.
Why Lower Unit Fitment Matters
A gearcase failure is one of the more expensive repairs a boat owner can face, and the parts involved aren't interchangeable across every Evinrude and Johnson outboard. Shopping for Evinrude Johnson lower units can get confusing fast, because several model families — SE304, SE305, SE306, SE308, SE316, and SE318 — cover different horsepower bands, case sizes, gear ratios, and rotation directions.
Ordering the wrong Evinrude lower unit replacement or Johnson lower unit replacement usually means paying for return shipping, waiting on a re-order, and sometimes eating a restocking fee — all before your boat is back in the water. This guide walks through how to identify what's currently on your engine, compares the SE304 through SE318 families side by side, and explains why horsepower alone won't get you to the right part.
How Do I Identify My Evinrude Lower Unit?
Before you can shop for a Johnson Evinrude replacement gearcase, you need three pieces of information from your own engine. Skipping this step is the single most common reason boat owners end up with the wrong part.
1. Find the Model and Serial Number Tag
Most Evinrude and Johnson outboards have a metal or foil tag mounted on the transom bracket or the engine mount, stamped or printed with a model number and serial number. This tag is your starting point — the model number narrows down the engine family, and the serial number pins down the exact production run, which matters because gear ratios and case designs changed within the same horsepower class over the years.
2. Inspect the Lower Unit Itself
If the tag is missing or unreadable, look directly at the gearcase. Note the physical case size (small, mid, or large), the shape of the skeg, whether there's a water pickup on one or both sides, and whether the propeller shaft appears offset for standard or counter-rotation. These physical traits help you narrow down which family — SE304 through SE318 — you're likely dealing with, but they should still be confirmed against the fitment chart rather than used as a final answer.
3. Count the Propeller Shaft Splines and Check Rotation
Spline count and prop shaft rotation (standard vs. counter-rotation) are two of the most overlooked details. Twin-engine setups often pair a standard-rotation lower unit on one side with a counter-rotation unit on the other, so don't assume both sides of a twin installation use the same part.
SE304–SE318 Lower Unit Comparison Chart
The table below gives a general overview of how the SE304 through SE318 families typically differ by case type, engine range, gear ratio family, and rotation. Use it as a starting reference point, not a final fitment decision — always confirm against your serial number before ordering.
| Model | Case Type | Typical Engine Range | Gear Ratio Family | Rotation | Notes |
|---|---|---|---|---|---|
| SE304 (90-304-A) | Small/mid case | Smaller inline-cylinder outboards | Standard ratio for this case class | Standard | Confirm exact case dimensions against your serial number before ordering. |
| SE305 (90-305-A) | Mid case | Mid-range inline or early V4 applications | Standard ratio for this case class | Standard | Often confused with SE304 due to similar case profile — check the tag closely. |
| SE306 V4 (90-306-C) | V4 case | V4 outboards | Early V4 2.0 or later V4 2.0 gear ratio, depending on production run | Standard | The V4 2.0 ratio family changed between early and later production — serial number determines which applies. |
| SE306 V6 (90-306-B) | V6 case | V6 outboards | V6 1.83 gear ratio family | Standard | Shares a model prefix with SE306 V4 but is a distinct casting — do not cross-shop by number alone. |
| SE308 (90-308-B) | V4 large case | Higher-output V4 outboards | V4 large-case 2.25 gear ratio | Standard | Physically larger case than the SE306 V4 — do not assume interchangeability. |
| SE308 CR (90-308-C) | V4 large case | Higher-output V4 outboards | V4 large-case 2.25 gear ratio | Counter-rotation | Built specifically for counter-rotation twin-engine setups — verify which side of the boat this is for. |
| SE316 (90-316-A) | V6 Magnum case | Higher-output V6 outboards | V6-specific Magnum ratio family | Standard | The "Magnum" case is physically larger than the standard V6 SE306 case. |
| SE318 (90-318-A) | V6 Magnum case | Higher-output V6 outboards | V6-specific Magnum ratio family | Counter-rotation | Counter-rotation counterpart to SE316 — confirm engine side before ordering. |
Table reflects general family characteristics only. Exact gear ratios, case dimensions, and year ranges vary by serial number — always cross-check with the manufacturer's fitment documentation before purchasing.
Is Horsepower Enough to Choose a Johnson Lower Unit?
No — horsepower is a useful starting filter, but it's not a reliable way to finalize your choice on its own. Evinrude and Johnson reused horsepower ratings across multiple engine families and production years, which means two outboards with the same horsepower badge can require completely different gearcases.
- Case architecture changed over time. A given horsepower class may have been produced with more than one case size or casting during its production run.
- Rotation isn't tied to horsepower. Standard and counter-rotation versions can exist within the same horsepower and family, especially on twin-engine boats.
- Gear ratio revisions happened mid-family. As noted in the comparison chart, the V4 2.0 ratio split into early and later versions, and horsepower alone won't tell you which one your engine needs.
- Regional and model-year variants exist. Small design changes were sometimes introduced without a change to the advertised horsepower.
This is why the model number and serial number — not the horsepower sticker — are the foundation of correct fitment. Horsepower narrows the search; the serial number confirms it.
What Gear Ratio Does My Evinrude Lower Unit Need?
Gear ratio describes how many times the crankshaft turns for each turn of the propeller shaft. A lower number means the prop spins closer to engine speed; a higher number means more mechanical reduction. Getting this wrong can affect how the engine loads under the propeller, so it's worth understanding the basic families referenced in the chart above.
Early vs. Later V4 2.0 Gear Ratio
Within the SE306 V4 family, Evinrude and Johnson produced an early V4 2.0 gear ratio version and a later V4 2.0 gear ratio version. Despite sharing the same "2.0" designation, these correspond to different production runs and aren't automatically interchangeable — this is exactly the kind of detail your serial number is used to confirm.
V4 Large-Case 2.25 Ratio
The SE308 counter-rotation family uses a V4 large-case 2.25 lower unit, built around a physically larger case than the standard V4 2.0 units. This larger case and different ratio are part of why SE308 isn't a drop-in swap for the smaller V4 families, even though both are V4 applications.
V6 1.83 Ratio
The SE306 V6 family runs on the V6 1.83 lower unit ratio, distinct from both the V4 ratio families and from the Magnum-case ratios used in SE316 and SE318. Because "V6" covers more than one case size across these families, ratio and case size need to be checked together rather than assumed from the cylinder count alone.
Frequently Asked Questions
Confirm Fitment Before You Order
The SE304 through SE318 families cover a wide range of Evinrude and Johnson outboards, and small differences in case size, gear ratio, and rotation direction separate one from the next. Before placing an order, take a few minutes to pull your engine's model and serial number and cross-check it against the manufacturer's official fitment chart. A few minutes of verification up front is the easiest way to make sure the lower unit that shows up at your door is the one your engine actually needs.