How To Detect A Blown Lower Unit On Outboard? (Explained)

The outboard lower unit, commonly known as the outboard gear case, plays a vital role in ensuring that your outboard is running smoothly and has long durability as well.

However, a blown lower unit does the total opposite of that and damages the outboard motor severely.

Today’s article will help you to understand symptoms that indicate a blown lower unit and how you can fix it with some easy and quick steps:

What Sign You Should Look For To Detect A Blown Lower Unit On Outboard?

There are numerous indications to give you hints of having a blown or damaged lower unit on your outboard. But if you are not aware of those symptoms beforehand, you may not notice until a major incident occurs.

The most common and earliest sign that you may notice is the operator shifting issues which mainly happens whenever any lower unit parts get damaged or affected. 

What Are The Symptoms Of A Blown Lower Unit On Outboard?

The other symptoms that indicate when you have a bad or blown lower unit on your outboard are usually related to these below indications:

Leakage Caused By Water in Gearbox Lube:

If you ever notice the water has started to mix with your outboard’s gearbox lube, it’s a clear indication that you have a leakage problem in gear lube, and you need to act quickly to fix that.

Otherwise, that destroyed gear lube can lead to damage to your lower unit furthermore.

Massive Metallic Contaminants On The Lower Magnetic Gearbox Drain:

Metallic contaminants going into the lower gearbox magnet are very obvious and unavoidable, but the amount is the main concern here.

In general, the metal particle amount should be no more than 3/4-inches on the outboard lower magnetic gearbox drain screw magnet.

If you notice a massive amount of metal particles in there and the metal is mixing with the process, that is a concerning situation to take care of because once the build-up contaminants amount becomes heavy, it gradually ruins your lower unit.

Trouble While Shifting Into Gears And Clanking Noises:

When you experience trouble during shifting into gears, or a loss of ability to turning the gears, it can happen when your outboard lower unit or any of its parts is in trouble.

If you closely observe, you will hear unusual steady clanking sounds too. This sound occurs when a tooth goes missing.

Eventually, it lodged between the gears when turned at the right RPM and may blow up the outboard lower unit.

These are the most common symptoms to identify whether you have a blown lower unit, which causes the major trouble you are experiencing with your outboard.

How Should You React If You Detect A Blown Lower Unit?

When you notice any possible indication of having a problem in your outboard lower unit, first stay calm and quickly follow the following instructions to replace your existing lower unit with a new one –

Turn The Engine Into Neutral & Removing the Prop, Gear Case:

For replacing the lower unit, first, remove the ignition switch key and pull the emergency lanyard.

Now, move your outboard engine control handle into the neutral position and use a socket wrench to remove the prop and gearcase.

Removing The Trim Tab:

To remove the trim tab, you first need to locate the splice connector at the gearcase’s top front side. Then, pull the two ends and remove the plastic plug located at the rear top of the gearcase.

Now you will get access to the bolt underneath that plastic plug so, use a socket wrench with an extender to release that bolt. Finally, remove the trim tab now.

Removing The Gearcase:

After removing the trim tab, you will see bolts where it was mounted so, remove those bolts for both sides. Now to remove the gearcase by pulling it straight out.

Next, wipe the driveshaft and shifter shaft splines into lithium grease to properly grease them.

Then, again check whether the control handle is in the neutral position or not.

 Replacing The Prob:

Next step is pushing the driveshaft upward into the motor’s middle part to reach the crankshaft. Be very careful during pushing the driveshaft and make sure crankshaft and driveshaft are in a realign position.

Now to insert the speedometer through the hole, lower the driveshaft a little bit. Then, push the gearcase up to embed a bolt finger there and tighten the bolt finger enough to maintain the alignment.

Now you will get free access to replace the prob, so replace the older one with a new set.

Testing The New Prob:

Now turn the control handle of your outboard engine into the forward and check the prob by trying to rotate it. It should not spin and then turn the control handle again into the neutral and try the same. Now the prob should spin freely without any trouble.

Similarly, turn your motor’s control handle into the reverse gear to check the prob once again, if the prob does not spin or move, it means the prob is working perfectly.

Finishing The Replacement Task:

Lastly, replace all the bolts and tighten all of them in the right positions. Then, push both splice connector’s ends on the speedometer cable to re-install the trim tab.

Next, replace the gear case and connecting all the bolts. Then secure all the connections properly and do a test run on your outboard engine to verify everything is set perfectly in places.

Is Replacing A Blown Lower Unit Typically Expensive?

Technically, the cost may vary depending on your outboard brand, motor type, and model, as well as the damage level.

If you only require a gear case replacement set, a standard set of gearcase parts can cost around $500 to $600, and the local labor cost will be approximately between $300 and $400.

If you need to change the shaft only, it may cost around $1000 excluding the labor pay.

But if you are pro in mechanical tasks and will be doing the replacement job all by yourself, it will not cost you much. You will just need to purchase the required parts and accessories, that is all.

How Often Should You Check Or Service Your Outboard’s Lower Unit?

In general, you should change your outboard lower unit oil and gear lube after every 100 hours of operation to prevent further damages.

However, the required servicing schedule may vary depending on your outboard brand, type, and model. Therefore, check your service manual for more accurate information.


The Outboard lower unit portion receives rotation and power from the engine’s powerhead, then transfers it to the propeller and prop shaft, so that the propeller can rotate or spin to move the outboard forward or backward through the water.

That is why you need to make sure that each part of your outboard lower unit is working properly.

Therefore, keep an eye on your outboard lower unit for any sign of fault to prevent further extreme damages.

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