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The motor in a MerCruiser-powered boat is bolted firmly to the floor and transom inside the boat. The outdrive is bolted firmly to the transom on the exterior of the boat. These two components are connected by a machined steel driveshaft that runs through a bearing in the gimbal housing and inserts into a coupler on the rear of the engine. If the alignment isn't perfect, the gimbal bearing may fail prematurely or the splines in the motor coupler will wear out.
Items you will need
Position the boat's shift lever in forward to allow the shift connection on the lower unit to separate cleanly from the connection on the boat.
Remove the lower unit from the rear of the boat by prying the cap off one end of the tilt/trim axle connection, removing the retaining clip with a flat screwdriver and removing the axle from the hydraulic cylinder ends. Remove the nuts that connect the lower unit to the gimbal housing with a ratchet wrench and separate the lower unit completely from the boat, pulling the driveshaft along with it.
Smear a thin layer of lithium grease on the machined end of an alignment tool.
Insert the alignment tool through the gimbal bearing and into the splined receiver on the engine coupler.
Make a mark on the alignment tool with a piece of chalk exactly on the top side of the handle part of the tool as an index.
Pull the alignment tool out of the rear of the boat and examine the greased end of the shaft. If the grease has been scraped off the machined end of the tool evenly all the way around the tool, the engine and lower unit are perfectly aligned. If the grease is scraped off the top of the shaft, the motor needs to be lowered slightly, if it's scraped off the bottom, the engine needs to be raised. If the grease is scraped off one side or the other, the engine needs to be moved a bit to the port or starboard to align things horizontally.
Adjust the engine by loosening the appropriate engine mounting nuts that secure the motor to the motor mounts. The motor mounts on the bottom of the engine control the up or down positioning of the motor; the mounts on the transom control the port and starboard attitude.
Turn the nut on the underside of the motor mount to raise, lower or shift the position of the motor as indicated, and then recheck the alignment with the alignment tool to see if more adjustments are needed. Once it's perfect, retighten the top engine mount nuts and reinstall the lower unit.
- A gasket and several O-ring seals make the outdrive watertight where it connects with the gimbal housing. Replace the old gasket and rings with new ones when reinstalling the lower unit.
- It's much more likely the engine will need to be adjusted higher or lower rather than horizontally to get a perfect alignment.
- Zubin Shroff/Digital Vision/Getty Images