Mercury Outboard Lower Unit Assemblies, Prop Shafts, and Transmission Components

Mercury Marine Engine Lower Unit Assemblies, Prop Shafts, and Transmission Components Resources and Information

Our complete selection of Mercury outboard lower unit assemblies, prop shafts, and transmission components are intended to replace your old or broken Mercury outboard parts. Need help performing maintenance on your Mercury outboard lower unit? Follow along with our Mercury Lower Unit 4-Stroke 100-Hour Service guide over on our blog.

We also regularly publish helpful resources to help you maintain your Mercury outboard motor, such as:

• Mercury Outboard Specs & Capacities Chart
• Mercury Outboard Serial Number Lookup
• Mercury Outboard 100-hour Service Guides

Mercury Outboard Lower Unit Assemblies, Prop Shafts, and Transmission Components FAQs

What is a Mercury outboard lower unit assembly? 

Also commonly called a gearcase, an outboard lower unit assembly transfers the rotational energy from the outboard engine and transfers it to the propeller, causing it to rotate and move the boat. The lower unit houses several essential components, without which you’re boat couldn’t move through the water.

How do I service my Mercury 4-stroke outboard lower unit? 

Use an appropriately sized screwdriver assisted by a wrench if necessary to loosen these screws which should be pretty tight. An impact driver and hammer might be needed. If the drain screw has a magnetic tip, inspect the plug after removal to see if heavy deposits exist. Some fine shavings are normal, but chunks or a large quantity of metal is cause for further inspection. Have a drain pan ready, and extra rags handy as gear lube will begin running down the skeg as soon as you remove the upper vent plug. Be sure to collect the used gaskets from both plugs after removal as these are not reusable and should be discarded.

Allow several minutes with the engine tilted all the way down to drain all the old gear oil fully. Like your engine oil, inspect the spent oil for evidence of water intrusion, other contamination, or metal. If you have purchased one of PartsVu’s handy lower unit oil change kits, you will already have new gaskets, oil, and a pump. Screw the pump into the lower drain hole, then connect it to the container of gear oil.

Begin pumping oil into the Mercury lower unit until oil begins to flow from the vent plug at the top. Stop pumping and wait a few minutes for all the air bubbles to work themselves out of the gearcase. After this brief pause, pump a few more times until clean gear oil without bubbles is flowing from the vent plug. With a fresh gasket in place, install and torque the top vent plug. Unscrew your fill hose from the lower drain plug and quickly swap in the lower drain plug with a new gasket — torque to spec.

Before you do get started on any maintenance projects, make sure to get the right parts (even for Mercruiser engines) so you don’t have to stop right in the middle.

How do I service my Mercury 2-stroke outboard lower unit? 

Use an appropriately sized screwdriver assisted by a wrench if necessary to loosen these screws which should be pretty tight. An impact driver and hammer might be needed. If the drain screw has a magnetic tip, inspect the plug after removal to see if heavy deposits exist. Some fine shavings are normal, but chunks or a large quantity of metal is cause for further inspection. Have a drain pan ready, and extra rags handy as gear lube will begin running down the skeg as soon as you remove the upper vent plug. Be sure to collect the used gaskets from both plugs after removal as these are not reusable and should be discarded.

Allow several minutes with the engine tilted all the way down to drain all the old gear oil fully. Like your engine oil, inspect the spent oil for evidence of water intrusion, other contamination, or metal. If you have purchased one of PartsVu’s handy lower unit oil change kits, you will already have new gaskets, oil, and a pump. Screw the pump into the lower drain hole, then connect it to the container of gear oil.

Begin pumping oil into the Mercury 2-stroke lower unit until oil begins to flow from the vent plug at the top. Stop pumping and wait a few minutes for all the air bubbles to work themselves out of the gearcase. After this brief pause, pump a few more times until clean gear oil without bubbles is flowing from the vent plug. With a fresh gasket in place, install and torque the top vent plug. Unscrew your fill hose from the lower drain plug and quickly swap in the lower drain plug with a new gasket — torque to spec.

How do I select the right parts for my Yamaha outboard motor?

PartsVu’s Mercury Outboard Maintenance Parts Library is the perfect reference when you need to replace common parts. That’s because these parts charts make it simple to find the right parts for your particular engine to perform regularly-scheduled maintenance. 

For Mercury outboard motors, perform maintenance after 100 hours of service or one year of operation (whichever comes first). More extensive service is recommended after 300 hours of service or three years.

After this period, maintenance is often more regular as the engine ages and parts wear out. For example, 1,000 hours generally requires an in-depth assessment by a qualified mechanic. Also, if you’re usually running your Mercury outboard in a more demanding manner, the manufacturer recommends more frequent servicing.

The following maintenance parts charts are organized by each model number for Mercury’s line of outboard engines.

How do I install a propeller on my Mercury outboard engine?

While many believe that installing a new propeller on a Mercury outboard is a tedious and cumbersome task, it is not the case. In fact, armed with the right knowledge, one can conveniently pull off the task of installing a new prop within a few minutes!

1. Using a screwdriver, push down the two bent-up tabs that are holding down the propeller nut.
2. Insert prop stop to prevent spinning.
3. Open lock nut retainer with Flat Screwdriver.
4. Remove prop nut.
5. Remove locknut retainer.
6. Remove drive sleeve adapter.
7. Remove drive sleeve from the boat propeller.
8. Remove forward thrust washer.
9. Clean any debris or old fishing line around the prop shaft.
10. Then inspect the shaft and analyze if it needs to be greased. If it does, coat a thin, even layer of Mercury 2-4-C grease on all the splines.
11. Select replacement propeller
12. Install forward thrust washer
13. Slide drive sleeve into the propeller
14. Insert drive sleeve adapter
15. Slide the new propeller on the prop shaft
16. Slide locknut retainer on prop shaft
17. Insert prop stop to prevent spinning
18. Tighten propeller nut
19. Finish with torque wrench, and tighten the nut further until you hear a click sound.
20. Close locknut retainer against locknut

How often should you change your prop?

In general, it is recommended to change a boat prop if one or more of the blade edges are chipped or broken by over 15%. In case the propeller is in good condition, and the blades haven’t suffered any damage, you can continue using the same without the need to change it.

How tight should a propeller nut be?

For a propeller nut to be accurately placed, it is crucial that the half prop nut is first installed and tightened to be just snug enough to preload it onto the prop shaft, against the hub. Next, the thick prop nut goes onto the shaft. All you need to make sure is that the propeller nut is tight enough, so as not to come off due to any vibration.

Is it easy to change a boat prop?

Yes, it definitely easy to change propellers. The steps are quite simple and usually can be completed within 30 minutes or so.

How often should I grease my Mercury outboard grease points?

You must grease all points at regular service intervals—minimally, annually. Saltwater boaters need to lubricate much more frequently as saltwater breaks down grease, and saltwater on non lubricated surfaces causes a great deal of damage.

Refer to your owner’s manual to locate all the grease fittings on your specific engine. Most will be easy to find, but a couple are likely tucked away and are not quite as visible. While you must familiarize yourself with the grease fittings on your specific engine, common grease fitting points include:

• Steering friction adjustment shaft (tiller handle models) 
• Swivel bracket
• Tilt tube
• Steering cable grease fitting

To add grease, first wipe off the grease fitting before attaching the grease gun nozzle to avoid pumping dirt and debris into the grease point. Also, you may have to remove rust and other debris clogging/blocking grease fittings. 

Pump grease into the fitting until grease begins oozing out of the exit points of the area being greased. The old grease will be pushed out as it is being dislocated by the new grease. Wipe off the old grease with your rag.

When a given grease point is full, leave a bit of grease on top of the grease fitting to keep rust and other debris from accumulating. Wipe off any excess grease with your rag, then move to the next grease fitting.

While not needed for all scenarios, it is worth noting that Mercury’s High Performance Extreme Grease is formulated for high temperature and high load applications and 100% compatible with lithium-based greases.

Which Mercury lower unit parts are most prone to failure?

There are several Mercury lower unit parts that are more prone to failure than others due to their location and exposure to stress. Here are some of the most common ones:

Mercury Lower unit seals

These are located between the upper and lower sections of the lower unit and can fail due to wear and tear or damage from impact.

Mercury Propeller shaft

This is a critical part that can become damaged due to corrosion, wear, or impact, which can lead to propeller vibration or even complete failure.

Mercury Water pump impeller

This is responsible for circulating cooling water through the engine and can wear out over time, especially if the engine runs in shallow or dirty water.

Mercury Gearcase bearings

These are located within the gearcase and are responsible for supporting the propeller shaft. They can become worn or damaged due to corrosion, impact, or lack of lubrication.

Mercury Shift shaft

This is responsible for engaging and disengaging the gears and can become damaged due to corrosion or impact.

Where can I learn more about Mercury outboard parts?

PartsVu understands that boaters want to learn how to perform their own Mercury outboard maintenance. From maintaining your Mercury outboard, Mercury sterndrive, and MerCruiser engines, there’s always something new to learn and save money. That’s why PartsVu offers our comprehensive blog, The Partsvu Xchange. 

Read up on the latest developments in boating, as well as search for everything you need to know about Mercury outboard maintenance.

To get you off to a good start, these are some of the most important articles on Mercury Outboard Lower Unit Assemblies, Mercury Outboard Propellor Shafts, Mercury Outboard Transmission Components and more:

• 11 Tips for Yamaha Outboards That Will Impress Your ...
• 7 Reasons Your Outboard Will Not Throttle Up and How ...
• Most Common Ways Your Yamaha Outboard Engine Will ...

What should I know about troubleshooting a Mercury 40hp outboard transmission?

When troubleshooting a Mercury 40hp outboard transmission, there are a few things to keep in mind. First, check the gear oil level and color. If the oil level is low, add more oil to the appropriate level. If the oil is dark or has a burnt smell, it may be time to change it. Next, check for any visible signs of damage or wear on the gears and clutch plates. If any of these parts are damaged, they will need to be replaced.

Another common issue with Mercury outboard transmissions is slipping or jumping out of gear. This can be caused by worn or damaged shift dogs or shift clutch components. To diagnose this issue, it's best to bring the motor to a qualified mechanic who can disassemble the transmission and inspect the internal components.

If you notice a grinding or whining noise when shifting gears, this may indicate a worn or damaged gear. In this case, the affected gear will need to be replaced. It's important to address transmission issues as soon as possible, as they can lead to further damage if left unaddressed.

Finally, it's important to follow the manufacturer's recommended maintenance schedule for your Mercury outboard engine. Regularly changing the gear oil and inspecting the transmission can help prevent issues from arising in the first place.

What is the lower unit assembly in a Mercury outboard motor?

The lower unit assembly is the part of the outboard motor that houses the gears, shafts, and propeller. It is responsible for transferring power from the engine to the propeller, which propels the boat through the water.

What is a prop shaft?

The prop shaft is the component that connects the lower unit assembly to the propeller. It transfers the rotational force from the engine to the propeller, which allows the boat to move through the water.

How often should I service my lower unit assembly?

It is recommended that you service your lower unit assembly at least once a year or every 100 hours of operation, whichever comes first. Regular maintenance can help prevent damage and prolong the life of your outboard motor.

What type of oil should I use in my lower unit assembly?

It is recommended to use a high-quality marine-grade gear oil specifically designed for use in outboard motors, such as Mercury's high-performance gear lube.

How often should I change the gear oil in my lower unit assembly?

It is recommended to change the gear oil in your lower unit assembly every 100 hours of operation or once a year, whichever comes first. Regularly changing the gear oil can help prevent damage and prolong the life of your outboard motor.

What is a transmission component in a Mercury outboard motor?

The transmission component is responsible for shifting the gears in the lower unit assembly, which controls the speed and direction of the propeller.

Can I replace my prop shaft myself?

It is recommended to have a professional marine technician replace your prop shaft. However, if you are experienced and knowledgeable in marine mechanics, you can replace the prop shaft yourself.

What is a lower unit seal kit?

A lower unit seal kit includes all the necessary seals and gaskets to replace the seals and gaskets in your lower unit assembly, which helps prevent water from entering the lower unit and causing damage.

How often should I replace the seals and gaskets in my lower unit assembly?

It is recommended to replace the seals and gaskets in your lower unit assembly every two to three years, or whenever you notice signs of wear or damage. Regular replacement can help prevent water from entering the lower unit and causing damage.

What is a Mercury prop shaft seal kit, and when should it be replaced?

The Mercury prop shaft seal kit is a set of components designed to prevent water from entering the gearcase through the propeller shaft. It should be replaced if there is any evidence of water intrusion into the gearcase or if the propeller shaft seal shows signs of wear or damage.

What is prop shaft end play, and how does Mercury prop shaft end play affect my outboard engine?

Prop shaft end play refers to the amount of axial movement that is present in the propeller shaft. Excessive end play can cause vibration, noise, and premature wear of the propeller shaft bearings. It is important to check and adjust prop shaft end play regularly to ensure proper operation and prevent damage to your Mercury outboard engine.

What type of Mercury prop shaft grease should I use on my Mercury prop shaft?

Mercury recommends using a high-quality marine-grade grease that is formulated specifically for use in outboard engines. It is important to use a grease that is compatible with the materials used in the propeller shaft and propeller hub to ensure proper lubrication and prevent damage to the components.

How do I remove the prop shaft from my Mercury outboard engine?

To remove the prop shaft from a Mercury outboard engine, you will need a Mercury prop shaft removal tool. This tool is designed to fit over the end of the propeller shaft and apply force to remove it from the gearcase. It is important to follow the manufacturer's instructions for using the prop shaft removal tool to prevent damage to the propeller shaft or other components.

What are the signs that I need to replace the prop shaft on my Mercury outboard engine?

Signs that you may need to replace the prop shaft on your Mercury outboard engine include excessive end play, vibration or noise from the propeller shaft, or damage to the propeller shaft or propeller hub. It is important to inspect the propeller shaft regularly and address any issues promptly to prevent further damage to your engine.

How do I replace the prop shaft seal on my Mercury outboard engine?

To replace the prop shaft seal on your Mercury outboard engine, you will need to remove the propeller and the propeller shaft. Then, you can access the prop shaft seal and remove it using a seal puller tool. Once the old seal is removed, you can install the new seal using a seal installation tool. It is important to follow the manufacturer's instructions for replacing the prop shaft seal to ensure a proper seal and prevent water intrusion into the gearcase.