The Truth about Gearcase Replacement

The essential part of the lower unit of an outboard motor is nothing more than the gearcase. The gearcase is equally known as the “business end” of the boat’s lower unit. When most boaters talk about repairing the lower unit is one of the most common and most expensive repairs boaters can encounter. It’s always important to purchase the correct lower unit parts needed for your repairs and replacements. If you are looking for a boat shop to work with, to purchase your gearcase, boat parts from PartsVu provide you best services.

Whatever you do, if you think you have damage to your lower unit but are hoping it will be okay. If you ignore the problem, know that internal damage to your unit may not be noticeable right away. Slightly damaged gears may seem to work fine at first but over time the off-kilter gears will wear away and cause eventual failure. We recommend you produce your motor be diagnosed thoroughly to determine if you produce internal damage.

What is the lower unit of an outboard motor?

An outboard motor’s lower unit is also called the gearcase. It is the part of the motor that gets flooded in the water. The outboard’s engine supplies power to the lower unit, which then uses this power to extract water through the system and turn the propeller. Both of these actions are essential for a properly functioning motor. The lower unit contains several key components, including:


 The driveshaft represents the part that transfers power from the engine, or powerhead, down to the gearbox and the water pump.

Shift shaft: The shift shaft controls the gear of the outboard motor. It controls your propeller by allowing you to shift forward and reverse.


The gearbox contains the actual gears that make shifting between forward and reverse possible. It also houses gears that connect to the driveshaft, converting its vertical power into horizontal power that moves the prop and prop shaft.


The propeller, as well as the prop shaft, represent equally essential components of the lower unit. The prop shaft moves the prop forward or backward giving to whatever gear the motor is operating within.

Water pump:

Most outboard motors take water to keep the engine cool. The water pump located in the lower unit extracts water into the motor and pumps it through the engine, preventing it from overheating. If any of these parts become damaged, the functionality of the entire motor may be compromised.

Can you run your boat without the lower unit?

If your lower unit is damaged or, for whatever reason, you want to try running your outboard without it, the engine will start up. But there are several reasons why it is a bad idea to run an outboard motor without the lower unit. Since the lower unit contains the propeller, it would be completely pointless to try and operate without it while you’re out on the water. Your boat isn’t going anywhere without that propeller. Since the lower unit is responsible for pumping water through the engine, operating without one will quickly cause the motor to overheat. It needs the water constantly flowing over it to wash away the excess heat it produces. In many outboard motors, the driveshaft pumps oil throughout the engine. Without the lower unit, you will have no driveshaft to provide the necessary pressure, and the engine will quickly size up if it isn’t receiving the oil it needs. In these cases, you should only run the motor for several seconds up to a couple of minutes. Turn it off as soon as possible. Get more consultancy by PartsVu (PartsVu is a leading boat parts company online).

When should you replace your lower unit?

1-Ensure that your screwdriver is large enough to remove the drain and vent screws without slipping and potentially ruining the screw slot–this will cause future removals and reinstalls tough. 2-Fill slowly; there’s no rush. Notice the solid flow of material coming out of the vent hole; no air bubbles mean it’s truly full. 3-Notice the expanded, distorted drain-screw gasket on the right; a modern one is on the left. These gaskets are cheap; replace them each time you adjust the gear lube. 4-After filling the unit so that the new material runs out of the upper vent hole, replace the upper vent screw; this produces a private vacuum so that when you produce the filler and replace the lower drain screw, you won’t lose much lubricant in the process. 5-Use a commercial filling canister if you have more than one or two units to maintain. You can purchase these through your local dealer.

How to replace a lower unit?

The first thing to do is to take the key out of the switch and pull the emergency lanyard, which is the line that clips to the operator’s 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 gearcase with a socket wrench. The next step would be to remove the gearcase by removing the bolt below the place where the trim tab was mounted and removing both bolts on either side. The gearcase will not come out easily. Now grease the drive shaft and shifter shaft splines by wiping them with a rag covered with grease. 

Gently pushing the drive shaft upward into the middle part of the motor, when it reaches the crankshaft. Finally, set the motor control handle to the forward position and then try to spin the prop. 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. Are you still confused to replace the lower unit?  PartsVu is a leading boat accessory to help you in this matter with free consultancy.


It is technically possible to run your outboard motor without the lower unit, but doing so could cause irreparable damage to the engine. This should prevent it from overheating or seizing up due to a lack of oil pressure. Most outboard motor problems that we have faced in boats are lower unit problems. lower units give out long before the upper units wear. Understanding them and treating them carefully helps lengthen their life, saving money and long trips to poling, paddling, or drifting home.

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