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2025-02-28
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The Truth About Gearcase Replacement
Table of Contents
Gearcase Replacement Course: Your Guide to Choosing the Best Option
Welcome to the Gearcase Replacement Course
I’m Mike Tracy, owner and founder of US Boatworks. Our mission is to help boaters select the best replacement gearcase available. Through this course, I aim to provide you with all the information necessary to choose a replacement gearcase that meets your specific needs effectively.
Why This Course Matters
Replacing a damaged gearcase is one of the most common—and expensive—repairs boaters face. The lower unit is particularly vulnerable to damage from hidden underwater objects.
By doing your homework before making a purchase, you can avoid the costly mistake of buying the first unit you find. Knowledge is power! Understanding your product and buying from a reputable company that knows your engine inside and out can save you hundreds, if not thousands, of dollars in the long run.
This is the first article in a four-part series. Each article explores one of three replacement options and provides additional tips to help you make the best decision.
Understanding the Components of a Lower Unit
To make informed decisions, it’s crucial to know the basics of gearcase components. In this section, we’ll introduce key terms and definitions related to the lower unit (gearcase), the part of your engine that sits below the waterline.
Gearcase or Lower Unit
- Also known as Gear Housing, Driveshaft Housing, Foot, or Lower Housing.
- Example: Volvo Lower Unit SX M #3855692, 1.43 ratio.
Upper Unit
- Contains the driveshaft connected through the transom to the engine, transmitting power to the gearbox. (Applies to sterndrive motors only.)
- Example: Volvo Penta SX A Upper Unit/Gearcase #3842919.
Complete Gearcase
- Refers to the combined upper and lower unit.
- Example: 5121600TP Complete Gearcase, Upper & Lower.
Gearbox/Gears
- The section of the lower unit where gears are located.
Propshaft or Driveshaft
- A mechanical component that transmits torque and rotation.
Splines
- Ridges on a driveshaft that mesh with grooves in a mating piece, transferring torque.
Clutch
- Connects and disconnects two rotating shafts, engaging or disengaging power transmission.
Skeg
- The lowest point of an outboard or sterndrive motor.
Types of Gearcases: OEM vs. Non-OEM
Choosing between OEM and Non-OEM gearcases comes down to balancing quality, compatibility, and cost—make an informed decision to ensure the longevity and performance of your boat.
OEM (Original Equipment Manufacturer):
- Manufactured by your boat’s engine maker (e.g., Mercury, Volvo-Penta).
- Built to exact specifications using original parts.
Non-OEM (Aftermarket):
- Produced by third-party manufacturers.
- Often labeled as “new” without explicitly stating they are aftermarket products.
Next Steps
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Now that you’re familiar with key terms, you’re ready for the next installment of this course, which dives into the details of new Non-OEM gearcases.
If you have boat insurance, contact your agent to see if your lower unit repair or replacement is covered under your policy.
Beth Tracy
I write about boat engine maintenance to help keep you out of the repair shop.
