The main part of the lower unit of an outboard motor is really nothing more than the gear case. The gear case is also known as the “business end” of the boat lower unit. When most boaters talk about changing the lower unit they are talking about disconnecting the speedometer cable, removing the gear case, and replacing the old, worn out gear case with a new or a rebuilt/remanufactured gearcase. It’s always important to purchase the correct lower unit parts needed for your repairs and replacements. Remember, the suggestions below are merely guidelines and your specific boat engine service manual should be consulted.
How to Replace a Lower Unit
The first thing to do is to take the key out of the ignition switch and pull the emergency lanyard, which is the line that clips to the operators life jacket out of the emergency stop switch. Next move the engine’s control handle into the neutral position. Now remove the prop and the gear case with a socket wrench.
Locate the splice connector which is comes out the top of the front side of the gear case and pull apart the two ends .On the top of the rear of the gear case there is a plastic plug, which you should pry out with a small screwdriver. Underneath that plug, there is a bolt; using a socket wrench, with an extender if necessary, loosen the bolt completely and remove the trim tab which is just above the prop.
The next step would be to remove the gear case by removing the bolt below the place where the trim tab was mounted and remove both bolts on either side. The gear case will not come out easily so it is easier to pry it open gently with a screwdriver ad removing the gear case by pulling it out straight. Now grease the drive shaft and shifter shaft splines by wiping them with a rag covered with lithium grease. Visually inspect the control handle to make sure that it is in neutral.
Gently pushing the drive shaft upwards into the middle part of the motor, when it reaches the crankshaft. When the crankshaft and drive shaft realigned, lower the drive shaft just enough to insert the speedometer through the hole line up the shift shaft, push the gear case all the way up and insert one bolt finger tight to maintain alignment. Replace the prop.
Finally, set the motor control handle to the forward position and then try to spin the prop. The prop should not turn. Return the control handle back into the neutral position; the prop should spin freely in both directions. Set the control handle to reverse. The prop should not move.
Replace and tighten all the bolts, reinstall the trim tab, push the ends of the splice connector on the speedometer cable together. Then replace the gear case cover, replace and tighten all bolts and test-run the motor.
Remanufactured lower unit is frequently used in the gear case replacement processes. Most shops use only quality parts with a good warranty, according to the manufacturer guidelines. They also include all internal parts and are fully integrated and interchangeable with OEM drives. Be sure to find a reputable shop that has been in business for many years. Check the warranty you receive on the unit which should be around one year. Do your homework before investing in a lower unit replacement as purchasing a cheap unit could leave you stranded out on the water.