Replacing a Damaged Lower Unit

Author: Chris Rossell

Here are the items we needed:

– 14 mm socket (and wrench)

– 12 mm socket

– Pliers

– Grease

– Gear lube and pump

– Torque wrench

– Large flathead screwdriver

If you boat long enough, you will eventually tag bottom, especially if you live in the Florida Keys. Earlier this year, we decided to go wakeboarding on our 23-foot Dusky center console, powered by a 2003 Yamaha HPDI 200. The place we always use for skiing is fairly shallow, but the depth is very consistent … or so we thought. Bam! Bam! Bam! Damn! And just like that, our skiing trip was done — just like the stainless steel prop, which was missing most of a blade.

When we replaced the prop, we could see it had some wobble from a bent propshaft. We checked around and found that replacing only the shaft would be close to $1,000 with labor, but other components could also be damaged. We looked at remanufactured lower units, but they cost $2,240. While our Yamaha runs great, that’s a lot of money to spend on a 10-year-old engine, since a good used one costs about $5,000. Then a friend of ours told us about SEI, a company in Oldsmar, Fla., that sells outboard and sterndrive lower units. When we discovered that a new replacement from SEI costs $845 and comes with a three-year warranty — and we could install it ourselves without any special tools — the decision became a no-brainer. Another advantage of doing this yourself is that you will be learning the same procedure for changing your water-pump impeller (enclosed in the white plastic housing on the new unit).

Within a couple of days, our SE421 lower unit arrived, and we went to work. Fortunately, SEI has a step-by-step instruction guide on its website (, in the Technical Guides section), so this is a project that’s very doable by anyone with decent mechanical skills.

Before starting work on the outdrive, shift the throttle lever into forward and remove the prop. Next, look on the top rear of the outdrive; remove the plastic access panel and use a 12 mm socket wrench to remove the bolt (photo 1) that holds the trim tab, which is used to minimize prop torque. After the trim tab is off, use a 14 mm socket to remove the bolt underneath (photo 2). Just in front of where the tab was mounted is another bolt that you remove with the same 14 mm socket.Replacing a Damaged Lower UnitBW_11-2013_DIY_02

Next, remove the water-pump hose attachment from the upper unit, then remove the six bolts that clamshell the upper and lower units together, using a 14 mm closed-end wrench to make sure you don’t strip the nut head (photo 3). A word of caution: If the bolts don’t come off easily, don’t apply too much force, because you can break the bolt off. To make removing the bolts easier, we tapped them a few times with a hammer, sprayed them with PB Blaster and let them sit. The new lower unit we got didn’t include bolts, so we reused the old ones, which were in good shape after being cleaned. We had to wiggle the lower unit back and forth a little to get the two sections apart, but it came off easily.BW_11-2013_DIY_03

Use pliers to turn the shaft counterclockwise until it stops, so you can shift it into forward to match the shifter, taking care not to damage the shaft splines. We used Red “N” Tacky to grease the driveshaft, taking care not to lube the very end, per the instructions. Grease any protruding pins as well as the water-pump hose hole before threading the hose through it.

Slide the lower unit carefully into place. There are alignment pins on the new unit, to make sure it seats properly, so make sure they line up (photo 4). If the splines of the new unit don’t seem to be matching up with the hidden gear of the upper unit, slip on a prop and gently spin it counterclockwise until it matches. Attach the water pump hose to the upper unit.BW_11-2013_DIY_05

Hand tighten all six connecting bolts. Then alternately tighten them with a 14 mm wrench (photo 5). If you have a torque wrench, all the bolts on this unit should be tightened to 29 pound-feet.BW_11-2013_DIY_06

Install the trim tab (which we recycled from the old unit) by first inserting the lower bolt and tightening it. Then install the other bolt just in front of it. Position the trim tab, and screw the upper bolt into place, then replace the plastic access cover.

Fill the gearcase with lube, which we got from SEI for $10. To do this, take off the upper and lower screws, and fill it from the bottom using the pump until lube seeps out of the upper hole (photo 6). Wipe everything clean and put in the upper screw. Unscrew the pump fitting from the lower hole (the lube won’t come out as long as the upper is in place), and install the lower screw.BW_11-2013_DIY_07

According to SEI, the break-in routine is very important. We varied our speed every five minutes and didn’t go above 75 percent throttle until after the first five hours of use. During those hours, we shifted into reverse more than 10 times. Then, between hours five and 10 we occasionally went full throttle. Between hours 10 and 20, we changed the gear lube. The next day, we were out hunting for lobster and fishing in the Gulf Stream, catching mahi mahi.

3 thoughts on “Replacing a Damaged Lower Unit

  1. How has the SEI lower unit held up? About 4 years later now. Would love to know how it’s done. Looks like I’m about to put one on my 300 HPDI.

  2. Just replaced the l/u on 150 Merc. $875 with 3 year aw s*** warranty. Drove to their shop which is amazing.
    On line chat if you have difficulties or any questions.
    Great product and great service


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