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The Truth About Gearcase Replacement

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The Truth About Gearcase Replacement

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Gearcase Replacement Course

Thanks for checking out my crash course on REPLACEMENT GEARCASES.

My name is Mike Tracy, the owner and founder of USBoatworks. We are a company on a mission to help boaters select the best replacement gearcase available. My goal for this course is to provide you with all the information needed to successfully choose the best REPLACEMENT GEARCASE to meet your specific needs. 

Do I Really Need To Read This?

Replacing a damaged gearcase is one of the most common and most expensive repairs boaters can encounter.  It’s a common repair because the lower unit is easily damaged by hitting hidden objects underwater.  There are lots of repair options available so doing your homework upfront and avoiding the temptation to slap on the first unit you find is smart. You are smart! Know your product and buy from a reputable company that knows your engine inside and out. Doing so can save you hundreds if not thousands of dollars down the road.

This is the first article in a series of 4. Each article discusses one of your 3 replacement choice then gives you a few more tips to help you decide what is best for you.

Components of a Lower Unit

In this first article, I will go over some definitions and lingo in the outboard and sterndrive arena.  

This image has an empty alt attribute; its file name is FourStroke_40-60hp_BigFoot_Gearcase-300x300.jpg

Knowing the basic components of your engine can be a big help when discussing a specific issues with your mechanic, insurance agent or friends. This diagram shows the inner workings of a gearcase.  For this class, we will be dealing with the lower unit components (gearcase), the part that sits below the water line.


volvo lower unit sx m #3855692, 1.43 ratio

Gearcase or Lower Unit can be called by many names: 

-Gear Housing  
-Driveshaft Housing  
-Lower Housing


volvo penta sx a upper unit/gearcase #3842919

Upper Unit – The upper contains the driveshaft connected through the transom to your engine which transmits power to a gearbox. (Sterndrive only)

5121600tp Complete Gearcase, Upper & Lower

Outdrive, drive unit, complete  –  describes the upper and lower unit combined. (Sterndrive only)

Gears/GearBox – section of the lower unit where the gears are located.

prop shaftsjpeg

Propshaft, propeller shaft or Drive Shaft –  mechanical component for transmitting torque and rotation, usually used to connect other components of a drive train that cannot be connected directly because of distance or the need to allow for relative movement between them.

Splines – ridges on a driveshaft that mesh with grooves in a mating piece and transfer torque.

Clutch – the clutch engages and disengages power transmission from driving shaft to drive shaft. Simply put, clutches connect and disconnect two rotating shafts drive shafts.

Skeg – the lowest point of an outboard or sterndrive motor.  

OEM – Original Equipment Manufacturer – that is the manufacturer that made your boat engine has also made the replacement part.  OEM parts made to the exact specs since the original manufacturer has the tools and specs to make this part. Examples of OEM Companies are: Mercury, OMC, Volvo-Penta.

NON-OEM –  aftermarket parts are those made by companies other than the OEM, which might be installed as replacements. Pay attention to how replacement products are labeled. Often times “Non-OEM” parts are simply listed as “NEW” but will not use the word aftermarket.


Now that you have all the definitions down pat, my next installment will dive into New Non-OEM gearcases. Here is the link to article 2.  

In the mean time, if you have insurance on your boat, contact your agent to see if your lower unit repair/replacement could be covered by your policy.


Mike Tracy

P.S. To get started with a new or remanufactured lower unit from USBoatworks, you don’t have to wait for the rest of this course; check out our website for all the models we carry.