Mercury Outboard Lower Unit Replacement Guide: Find the Right Gearcase for Your Engine | Mariners Warehouse
Mercury Outboard Lower Unit Replacement Guide: Find the Right Gearcase for Your Engine
A plain-English walkthrough of Mercury's lower unit lineup — organized by horsepower, shaft length, and gear ratio — so you order the gearcase that actually bolts up and turns your prop the right way.
Shop Mercury Lower Units →A lower unit — or gearcase — isn't a universal part. It's built around three numbers that have to match your engine exactly: horsepower, shaft length, and gear ratio. Get any one of those wrong and you're looking at a housing that won't line up with your midsection, a prop shaft spinning at the wrong speed for your power head, or — on twin-engine boats — a gearcase spinning the wrong direction entirely.
Horsepower and model year narrow you down to a family of gearcases. Shaft length (usually stamped as 20", 25", or 30", sometimes shown as standard, Long, XL, or XXL) has to match the distance from your transom to the waterline. Gear ratio determines how many times your engine's crankshaft turns for every one turn of the propeller — swap ratios and your engine can end up over-revving or laboring under load, even if the housing physically fits. Below, we've grouped Mercury's lower unit catalog by HP range so you can go straight to the section that matches your outboard, then confirm the shaft and ratio against your engine tag before you order.
40–90 HP Small Block & Bigfoot Models
This range covers the small-block 3- and 4-cylinder Mercury outboards, including the wide, high-thrust Mercury Bigfoot 40–90HP lower unit family that trades top speed for extra low-end pushing power on pontoons and larger tenders.
SE90-205
Fits 3- and 4-cylinder 2-stroke Carb/DFI and 4-stroke powerheads, and doubles up as the correct case for 40–60 HP Bigfoot models.
Bigfoot Lower Unit — 4-Cylinder 4-Stroke
The dedicated Bigfoot housing for 4-cylinder 4-stroke outboards, available new or remanufactured.
Bigfoot Lower Unit — 3-Cylinder
The earlier-generation 3-cylinder Bigfoot gearcase for older mid-power outboards.
Outboard Lower Unit — 4-Cylinder 4-Stroke
Standard (non-Bigfoot) gearcase for 4-cylinder 4-stroke outboards in this power band.
Outboard Lower Unit — 3-Jaw & 6-Jaw Clutch
Covers older 3-Jaw and 6-Jaw clutch designs across a wide production run — check your existing prop shaft clutch style before ordering.
Not sure which one fits?Send us your engine's HP, model year, and shaft length and we'll confirm the right part before you buy.
Ask Our Team110–150 HP Mid-Range
Mid-range Mercury 4-cylinder and early V6 outboards use a handful of long-production gearcases, most of which were built to outlast several engine generations.
Outboard Lower Unit — 110–125 HP L4
3-Jaw and 6-Jaw clutch versions covering nearly two decades of L4 production.
Outboard Lower Unit — 4-Cylinder
Built for the 2001–2003 115 HP 4-cylinder, and also correct for 75–115 HP 4-cylinder outboards built 2007–2014.
Outboard Lower Unit — V6, 2:1 Ratio
A long-running V6 gearcase with a straightforward 2:1 gear ratio, spanning three decades of production.
150–250 HP V6, Optimax & DFI
This is the largest family in the catalog, covering Mercury's 2.0L through 3.0L V6 blocks. If your engine is a direct-injection Optimax or DFI model, this is also where you'll find the Mercury Optimax V6 lower unit built specifically for that fuel system's power and torque curve.
SE90-206
Fits Mercury's 2.0L, 2.4L, and 2.5L V6 blocks.
SE90-216
The 3.0L V6 gearcase, offered in the widest shaft selection in the collection, including an Alpha-style option.
Outboard Lower Unit — 3-Cyl DFI/Optimax
Purpose-built for 3-cylinder direct fuel injection and Optimax powerheads in this HP band.
Outboard Lower Unit — V6 3.0L
Standard V6 gearcase for this displacement and power range, spanning nearly forty years of engine production.
Outboard Lower Unit — V6 3.0L
The higher-output version of the same 3.0L V6 architecture, in the largest shaft options offered.
Browse the full rangeSee every Mercury V6, DFI, and Optimax gearcase currently in stock.
View CollectionVerado & Pro XS High-Performance
Mercury's supercharged and high-output platforms need their own gearcases. If you're running a supercharged 4-cylinder or V6 Verado, you'll want the Mercury Verado lower unit below; racing- and bass-boat-oriented owners running a high-thrust or performance drive should look at the Mercury Pro XS lower unit entry.
SE90-221
The 4.8" bullet-diameter Verado gearcase, offered in both 2-stroke and 4-stroke configurations. It's also the correct case for 135–200 HP supercharged 4-cylinder Verado engines built 2006–2019.
Verado Lower Unit — 4-Cyl Supercharged
Dedicated gearcase for the supercharged 4-cylinder Verado platform.
Outboard Lower Unit — PRO XS & Command Thrust
Built for Mercury's performance-tuned PRO XS models as well as Command Thrust high-thrust configurations.
Counter Rotation for Twin-Engine Setups
Twin-engine boats need one standard-rotation and one counter-rotation gearcase so both propellers turn against each other, canceling out torque steer. If you're replacing the port- or starboard-side unit on a twin install, you need a genuine counter rotation outboard gearcase — a standard-rotation housing will not work here, regardless of HP match.
SE90-218 — Counter Rotation
The counter-rotation companion to the SE90-216, sharing the same 3.0L V6 displacement and ratios.
SE228
Replaces select counter-rotation 3.0L V6 units, as well as 2.6L I6 and 1.7L I4 applications.
150HP 4-Stroke Lower Unit — Counter Rotation Conversion
A counter-rotation conversion case for F150 PRO XS and F150 SeaPro platforms.
Counter Rotation Lower Unit — V6 3.0L
The counter-rotation match to the 200–250 HP 3.0L V6 standard-rotation gearcase above.
Still narrowing it down?
This covers the core of Mariners Warehouse's Mercury lower unit inventory, grouped by horsepower so you can match your engine tag against the specs above. The full Mercury collection includes additional variants beyond this list — browse everything currently in stock, or send us your engine's HP, year, and shaft length and we'll point you to the exact match.