Counter Rotation Lower Unit 1.85 – Johnson/Evinrude Magnum & M2 | 5007670

A blown Magnum or Mag 2 gearcase on a counter rotation motor puts you in a bad spot. Used counter rotation cases are scarce and usually come with no history and no warranty — the ones floating around eBay are typically pulled off high-hour motors, sold with corrosion noted and “no warranty” in the description. A full rebuild means paying a shop to tear it down, shim it, and hope the housing and shafts are still within spec. This is the third option: a brand-new aftermarket case that bolts into the same place your old one came out of.

The SE318 is built by Sterndrive Engineering (SEI), who have been producing replacement outboard lower units since 2008 and assemble them at their Florida facility from components made to their own specifications. SEI’s position on remanufactured drives is blunt — they don’t build them, because reusing worn cases, gears, shafts and bearings makes remaining service life impossible to judge. Everything in this box is new.sterndrive+1

What it fits

This is a direct replacement for the Mag II (M2) counter rotation gearcase, 1.85:1 gear ratio. Published fitment tables cover Johnson and Evinrude V6 models from 2002 through 2012 plus the AA and AB model-year families, in 135, 150, 175, 200, 225, 250 and 300 HP, across 20″, 25″ and 30″ applications. Later 3.3L and 3.4L E-TEC counter rotation motors are included in that range.operatorsguides.brp+1

Match your engine model number before you buy. Not “225 HP” — the full alphanumeric code off the motor, like E225DCXABF or DE300CZAAC. Counter rotation cases are also easy to confirm on the motor itself: BRP stamps an “L” into the end of the propeller shaft on left-hand rotation gearcases, and factory counter rotation motors carry a counter rotation decal on the rear cowl. If your prop shaft isn’t stamped L, you need the standard rotation case, not this one.

Older engines that originally left the factory with the first-generation Magnum case (roughly 2002–2005 model codes in the fitment tables) can also run this unit, but with one extra step — see below.

Read this before you order — the parts that are NOT in the box

Being straight about this saves you a second shipment and a lost weekend. This unit does not include:

• Upper drive shaft

• Shift shaft

• Water pump

• Water intake screens (reuse the ones off your original gearcase)

Most applications reuse the existing upper drive shaft, so you don’t have to buy one. If yours is damaged, the correct SEI upper for 90° V6 25″ applications is 92-306-04-25B and for 60° V6 25″ applications it’s 92-306-04-25C. 30″ applications are not covered by an SEI shaft — the upper for those is OE# 5007673.seimarine+1

If you’re replacing an original Magnum (not a Mag 2)

The SE318 ships with alignment pins fitted for M2 mounting. Going onto an engine that came with the earlier Magnum case, you need to remove the alignment pins from the new lower unit before it will seat. It’s a five-minute job, but skip it and the case won’t pull up flush.sterndrive+1

Water pump kits — the 3.4L exception

SEI’s 90-306-01K pump kit is not a direct fit on 3.4L engines. The correct kit for 3.4L is OE# 5007972, which is a BRP part. That kit is documented by BRP as the water pump kit for outboards using a 5/8″ internal-diameter water tube and is catalogued for 200–300 HP 3.4L models from 2009 up.marineengine+4

If you’d rather run the SEI kit on a 3.4L, three things have to be done:

1. Do not install the water tube grommet in the housing.

2. Reuse the O-rings from your old water tube.

3. Reuse the cover from your water pump housing — the plastic tube held on by two screws.

Finish

Black only. There is no white, blue, or grey version of this case. If you’re running a white E-TEC cowl, plan on either living with the contrast or painting it.

Warranty — what “3 year fault free” actually means

SEI covers assembled outboard lower units with a three-year fault-free warranty, which covers any failure requiring repair or replacement, not just manufacturing defects. That’s a genuinely unusual level of coverage in this category, and it’s the main reason these units get recommended on boating forums — owners report calling SEI, giving the serial number, and getting the unit rebuilt or replaced under the no-fault terms.thebassbarn+3

The limits are real and worth knowing up front, straight from SEI’s warranty document:

• Coverage starts on the shipment date, or on the date of purchase from an authorised SEI dealer. Keep your receipt — proof of purchase is required.

• Void if fitted to the wrong application. This is why matching your engine model number matters.

• Does not cover racing or competitive use, commercial/rental use, or use above SEI’s stated horsepower limits.

• Void if the model or ID numbers are altered or removed, or if the unit is disassembled without SEI’s authorisation.

• Claims must be submitted in writing within 14 days of discovering the fault, and the unit returned within 30 days of first notice.

• Liability is capped at the purchase price; labour and consequential costs aren’t covered.

Installation and break-in

Plan on reusing your existing gearcase mounting bolts — new units don’t ship with them. Note your shift rod height in neutral before you unthread it, because you’ll set the new one to the same measurement. Pulling the upper driveshaft out of the old case is the one job that usually needs a proper driveshaft puller. One easy-to-miss detail from BRP’s own service manual: standard rotation and counter rotation trim tabs are not interchangeable — don’t swap one on by mistake.boatingmag+3

SEI’s break-in routine matters if you want the warranty to mean anything:

• First 5 hours: vary your speed roughly every five minutes and stay under 75% throttle. Shift into reverse more than 10 times during this period.

• Hours 5–10: occasional full throttle is fine.

• Hours 10–20: change the gear lube.

Not sure it’s your gearcase?

Before you spend on a lower unit, pull the drain and vent plugs and look at what comes out. Milky, grey or coffee-coloured lube means water is getting past a seal. Fine grey fuzz on the magnetic drain screw is normal wear; actual flakes or chunks that feel metallic when you rub them mean a bearing is coming apart or a gear tooth is gone. Whining or howling that gets worse under load points at gears; a rumble that just tracks RPM points more at shafts and bearings. Popping out of gear under load, notchy shifting, or a case that binds when you turn the prop shaft by hand are all reasons to stop running it.bornagainboating+5

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