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. Sometimes after hitting an object underwater, the damage may not be totally obvious. The problem is that a small bend in your components can cause serious damage down the line. Have your marine mechanic check out your lower unit for internal damage sooner rather than later. Also, get your insurance company involved as they may cover some of this expense.
Once you find that you have lower unit damage, you will quickly discover that there are lots of repair options available. Doing your homework upfront and avoiding the temptation to slap on the first unit you find is smart. Know your product and buy from a reputable company that knows your engine inside and out.
To know your product and really understand the importance of purchasing a quality lower unit, you must first understand the components of your gearcase. I will address the components of a gearcase in this article then in the following articles, I will cover replacement and repair choices with the pros and cons of each.
Knowing the basic components of your engine can be a big help when discussing a specific issues with your mechanic, insurance agent or friends. The diagram below shows the inner workings of a gearcase. For this article, we will be dealing with the lower unit components (gearcase), the part that sits below the water line.
A Gearcase or Lower Unit can be called by many names:
Gear Housing
Driveshaft Housing
Foot
Lower Housing
- Upper Unit – The upper contains the driveshaft connected through the transom to your engine which transmits power to a gearbox. (Sterndrive only)
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.
- 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 driven 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.
Now that you have all the definitions down pat, my next article will dive into New Non-OEM gearcases. If you want to read ahead, here is the link to my next gearcase article.
Mercuriser Uppers in stock, ready to ship. 
Mercruiser Uppers on the bottom rack and Mercruiser Lower Units on the top rack.
Evinrude and Mercury Lower Units racks are found.
OMC Lower Unit parts & Volvo Sterndrive Gear Case components 
The pictures above are a small sampling of the huge lower unit inventory at US Boatworks. Visit our Lower Units page to buy Mercruiser Lower Units, Mercury Lower Units, Evirnude Lower Units, Johnson Lower Units, Mariner Lower Units, Force Lower Units, and Volvo Penta Lower Units.
