Replacing a Yamaha Outboard Lower Unit

When you finally hit bottom — and you will — you’ll want to know how to install a new Yamaha lower unit. 

Tools Required:

- 14 mm socket (and wrench)

- 12 mm socket

- Pliers

- Grease

- Gear lube and pump

- Torque wrench

- Large flathead screwdriver

Marine Parts Warehouse is a dealer for Sterndrive Engineering. We sell new replacement SEI lower units for $845.00 and they come with a three-year warranty — and you can install it yourself without any special tools.

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 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.

Just in front of where the tab was mounted is another bolt that you remove with the same 14 mm socket. 

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.

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, tap them a few times with a hammer, spray them with PB Blaster and let them sit. The new lower unit will not include bolts, so reuse the old ones. Wiggle the lower unit back and forth a little to get the two sections apart. 

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. 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

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.

Hand tighten all six connecting bolts. Then alternately tighten them with a 14 mm wrench.

If you have a torque wrench, all the bolts on this unit should be tightened to 29 pound-feet.

Install the trim tab (which can be 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. 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

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.

According to SEI, the break-in routine is very important. Vary your speed every five minutes and didn’t go above 75 percent throttle until after the first five hours of use. During those hours, Shift into reverse more than 10 times. Then, between hours five and 10 occasionally go full throttle. Between hours 10 and 20, change the gear lube.