Every SEI lower unit is a direct replacement for a specific Mercury OEM gear housing. The reliable way to find yours is to identify your Mercury complete gear-housing part number first, then cross-reference it on our site.

1. Find your engine serial number — it's on the engine's ID/serial tag (transom bracket or powerhead). This is the key that unlocks everything else.
2. Confirm your exact engine model and year on Mercury's owner's-manual page.
3. Open Mercury's Parts Finder and select your engine. Go to the Gear Housing (lower unit) section for your motor.
4. Pick the right part number. The gearcase diagram lists dozens of numbers — seals, bearings, gears, the water pump, individual housings. Ignore those. You want the complete gear housing / lower unit assembly number for the whole unit. If more than one complete assembly is listed, match it to your shaft length, gear ratio, and rotation.
5. Type that OEM assembly number into the search box at the top of sterndrive.cc and follow the result to the matching SEI lower unit.
6. Confirm the shaft length and gear ratio on the SEI product page match your engine before you order.

Mercury tacks a letter code onto the end of the model (like a “250XL” or “300CXL”) that tells you the shaft length and rotation:

• L (Long) = 20″ shaft
• XL (Extra Long) = 25″ shaft
• XXL (Extra Extra Long) = 30″ shaft
• CXL = Counter-rotation, 25″ shaft
• CXXL = Counter-rotation, 30″ shaft

The “C” always means counter-rotation — the left-hand engine on a twin setup. If your motor has a C in the code, you need a counter-rotation SEI unit; a standard-rotation unit will not work. When in doubt, measure from the mounting surface of the gear housing to the top of the driveshaft splines: roughly 23″ = 20″ shaft, 27–28″ = 25″ shaft, 32–33″ = 30″ shaft. And watch for a spacer — a 25″ shaft with a 5″ spacer can look like a 30″.

Not every Mercury gearcase has an SEI replacement, and it comes down to the propeller shaft:

• FleetMaster (the standard gearcase) uses a 15-spline prop shaft. This is the one SEI makes — our Mercury lower units replace FleetMaster-style gearcases.
• TorqueMaster, TorqueMaster 2, and Sport Master are Mercury's heavy-duty / high-performance gearcases, used on many Pro XS and high-output motors. They use a bigger 19-spline “fat shaft” propeller and a more robust case. SEI does not manufacture a TorqueMaster or Sport Master unit. A standard SEI gearcase can physically bolt up to some of these, but the prop shaft is smaller (it needs a hub-kit change) and it's not a warrantied replacement — Mercury built those cases heavier for a reason, so we won't pretend ours is an equal swap.
• The 5.44″ gearcase (the big “fat-shaft” case on higher-horsepower Verado — 275, 300 L6, 350, including the 5.44″ Sport Master) is likewise not made by SEI. We build the smaller 4.8″ case used on 200–300 hp Verado applications. Our 4.8″ unit may physically fit a 5.44″ application, but again, only without warranty.

If your Parts Finder lookup shows a 19-spline fat shaft, a TorqueMaster/Sport Master designation, or a 5.44″ gearcase, don't order a standard unit blind — call or text us first at 813-925-7127 (Mon–Fri 8am–5pm ET) with your serial number and we'll tell you honestly whether we have a fit.