MerCruiser Alpha One Gen I complete standard-rotation sterndrive

Alpha OneĀ® Gen I Sterndrive Lower Applications

Shop aftermarket Alpha One Gen I lower units in standard and counter-rotation configurations. Compare the SE106 and SE108 lower gearcases, then confirm your existing drive family and rotation before ordering.

Alpha OneĀ® Gen I Sterndrive Lower Applications

Shop MerCruiser Alpha One Gen I lower-only gearcases (1972-1990) ā€” replacement lower units for boaters who already have a working upper gear case and only need the lower section replaced due to wear, corrosion, or impact damage.

Pair this Alpha One Gen I lower unit with your existing upper section for a cost-effective partial rebuild.

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  • Lower Unit | SE106 | Mercruiser Alpha One (1972-1990)

    Lower Unit | SE106 | Mercruiser Alpha One (1972-1990) SKU: SE90-106-02
    MPN: 90-106-02

    $1,065.00
    $1,182.15
    $1,065.00
    Restore your stern driveā€™s propulsion with this Standard Rotation Lower Unit, a premium aftermarket replacement for Mercruiser Alpha One (Gen I), R, and MR models. Engineered for reliability in both freshwater and saltwater, this unit features a 13-spline drive shaft and precision-machined 13-21 tooth...
    $1,065.00
    $1,182.15
    $1,065.00
  • Counter Rotating Lower Unit | SE108 | Mercruiser Alpha One

    Counter Rotating Lower Unit | SE108 | Mercruiser Alpha One SKU: SE90-108-02
    MPN: 90-108-02

    $1,295.00
    $1,437.45
    $1,295.00
    Eliminate steering torque and improve tracking with this Counter Rotating Lower Unit, a premium aftermarket replacement for Mercruiser Alpha One (Gen I) drives. Engineered for twin-engine applications, this unit features a high-torque gear set and heavy-duty internal components designed to deliver superior acceleration and...
    $1,295.00
    $1,437.45
    $1,295.00

A Clearer Way to Select Your Replacement Drive

Compare the configuration details that matter before choosing a sterndrive.

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Coverage on select lower units

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How to Choose the Correct Alpha One Gen I Lower Unit

Start by identifying the drive that is installed now. The two Alpha One Gen I lower-unit options on this page are separated primarily by rotation: the SE106 is the standard-rotation replacement, while the SE108 is the counter-rotation replacement.

Similar appearance, boat year, engine horsepower, or drive position does not prove compatibility. Record the existing drive and engine identifiers, then compare them with the exact product page.

The SE106 manufacturer listing specifies R, MR, and Alpha One replacement coverage, a 13-spline driveshaft, and a 13ā€“21 tooth count. The SE108 manufacturer listing states that it replaces counter-rotation units only.

If the installed drive has been changed, converted, or assembled from mixed components, request Alpha One lower-unit fitment assistance before checkout.

Verify These Details Before Ordering

āœ“ Confirm the installed drive family and generation.

āœ“ Record the engine model and serial number.

āœ“ Record the drive serial number and visible part or casting numbers.

āœ“ Confirm standard or counter rotation.

āœ“ Do not select the lower unit from boat position alone.

āœ“ Compare the installed unit with the exact SE106 or SE108 product page.

āœ“ Confirm compatibility with the retained upper assembly.

āœ“ Check which installation parts, lubricant, hardware, and accessories are included or sold separately.

SE106 vs SE108 Alpha One Lower-Unit Comparison

Use this table to compare the two Alpha One Gen I lower-unit configurations currently represented in the collection. It is a shopping guide, not a universal compatibility chart. Final fitment must be confirmed from the installed drive information and selected product page.

SE106 Standard Lower Gearcase SE108 Counter-Rotation Lower Gearcase
Standard / right-hand rotation Counter / left-hand rotation
Manufacturer model: SE106 Manufacturer model: SE108
SKU / MPN: 90-106-02 SKU / MPN: 90-108-02
SEI lists replacement coverage for supported R, MR and Alpha One applications SEI lists this as a counter-rotation replacement onlyā€”not for standard-rotation applications
13-spline driveshaft, 13ā€“21 tooth count and OE reference 8951A43 Counter-rotation configuration with OE reference 8951A33
Verify the drive family, generation, rotation and installed identifiers Verify the counter-rotation requirement, installed identifiers and conversion history
Confirm retained upper-unit compatibility and included components Confirm retained upper-unit compatibility and included components

Do not substitute a standard-rotation lower for a counter-rotation unitā€”or the reverseā€”because the housings appear similar. Confirm the installed drive, rotation, and product-specific application or request fitment assistance before purchasing.

Lower Unit, Upper Unit or Complete Sterndrive?

An Alpha One lower unit, upper unit, and complete sterndrive solve different repair needs. Confirm which assembly has failed before purchasing a replacement.

Lower Unit

  • The lower gearcase portion located beneath the upper drive housing.
  • Suitable when inspection confirms that damage is isolated to the lower assembly.
  • The retained upper unit must remain serviceable and compatible.
  • Usually does not include the upper unit, transom assembly, propeller, installation hardware or required fluids unless stated on the product page.

Upper Unit

  • Contains the upper driveshaft housing and its related internal components.
  • Suitable when the failure is isolated to the upper assembly.
  • The existing lower gearcase must remain serviceable and compatible.
  • Usually does not include the lower gearcase, propeller, transom assembly or installation components.

Complete Sterndrive

  • Combines a matched Alpha One Gen I upper unit and lower gearcase assembly.
  • Suitable when both assemblies are damaged, contaminated or mismatched.
  • May be the better option when the existing drive components are difficult to identify.
  • Usually does not include the transom assembly, propeller, trim system, installation kit, lubricant or accessories unless stated on the product page.
An informational comparison graphic titled 'STANDARD VS COUNTER ROTATION' showing two outboard motors and two propellers, with text explaining the difference in propeller direction when viewed from behind. The background is a real marina scene with water and boats, but the image functions as an explanatory diagram rather than a straightforward product photo.

Standard vs Counter-Rotation Alpha One Lower Units

The SE106 is a standard-rotation or right-hand Alpha One Gen I lower unit for supported MerCruiser R, MR and Alpha One applications. The SE108 is a counter-rotation or left-hand lower unit and must only replace an existing counter-rotation gearcase. These Alpha One replacement lower units are not interchangeable.

Before ordering, confirm the installed drive family, generation, rotation, serial or part numbers and compatibility with the retained upper unit. On twin-engine boats, port or starboard position alone does not prove rotation; verify the propeller direction and installed drive information or request professional fitment assistance.

RELATED MERCRUISER COLLECTIONS

Continue Shopping MerCruiser Sterndrive Replacements

Need a different assembly or generation? Browse the broader Alpha One Gen I drive hub, compare complete standard-rotation sterndrives, review complete counter-rotation options, or move to the separate Alpha One Gen II lower-unit collection.

Keep each selection focused on the installed drive family, generation, rotation, and assembly level so similar-looking MerCruiser sterndrive replacements are not treated as interchangeable.

  1. Alpha One Gen I Parts and Assemblies

    Browse the broader Alpha One Gen I collection for available sterndrive assemblies, lower-unit components, replacement parts, and related applications.

    Explore Alpha One Gen I

  2. Complete Standard-Rotation Sterndrives

    Compare complete Alpha One Gen I standard-rotation sterndrives across the available ratio and input-yoke configurations.

    Shop Standard Sterndrives

  3. Complete Counter-Rotation Sterndrives

    Explore complete Alpha One Gen I counter-rotation sterndrive configurations for supported twin-drive replacement applications.

    Shop Counter-Rotation Drives

  4. Alpha One Gen II Lower Units

    Shop the separate Alpha One Gen II lower-unit collection for supported 1991-and-later-style applications, including listed standard and counter-rotation options.

    Shop Gen II Lower Units

Replacement Lower Gearcases for Alpha One Gen I Applications

This collection brings together the two aftermarket lower-only configurations currently offered for supported MerCruiser Alpha One Gen I-style applications. The SE106 represents the standard-rotation path, while the SE108 represents the counter-rotation path.

Each product is a lower gearcase assemblyā€”not a complete sterndrive, upper unit, transom assembly, or universal replacement. Use this collection to compare assembly type and rotation, then review the individual product pages for verified specifications and current availability.

R, MR, Alpha One Gen I, and Alpha One Gen II should not be treated as interchangeable drive names. Previous repairs or conversions can also change what is currently installed on the boat. Record the existing serial and part numbers and confirm the installed configuration before purchasing an Alpha One Gen I lower unit.

What Customers Say

Perfect Spline Match for DIY Swap

Great value replacement for my old Alpha One lower unit. The 13-spline shaft on the 90-106-02 matched perfectly with my drive. Highly recommend for anyone doing a DIY swap.
Tom Whitfield Verified buyer
August 2026

Torque Steer Eliminated on Twin Sterndrives

Upgraded both drives with the SE106 and this SE108 counter rotator (90-108-02). Torque steer is basically gone now ā€” great match for twin sterndrive boats.
Brian Hutchins Verified buyer
August 2026

Improved Twin-Engine Tracking

Product (90-108-02) does what it says, though shipping took a little longer than the estimate. Once installed, twin-engine tracking was much better.
Angela Reyes Verified buyer
April 2026

Solid Counter Rotating Unit

Solid build quality on this counter rotating unit, MPN 90-108-02. Used it to replace a worn-out gear set and acceleration on my starboard engine noticeably improved.
James O'Connell Verified buyer
June 2026

Fast Arrival, Flawless Fit

Ordered the 90-106-02 lower unit as a replacement for a cracked housing. Packaging was solid, arrived faster than expected. Only minor gripe is the paint finish wasn't perfectly matched to OEM, but mechanically it's been flawless.
Rachel Ordonez Verified buyer
July 2026

Helpful Guides

Use these guides to identify your MerCruiser drive, understand gear-ratio selection and decide whether you need a complete sterndrive, upper unit or lower unit.

MerCruiser Alpha One Gen I standard rotation and counter rotation lower unit gearcases side by side on a workbench

SE106 or SE108: Which Alpha One Lower Unit Do You Need?

SE106 or SE108: Which Alpha One Lower Unit Do You Need? Choosing the correct Alpha One Gen I lower unit...

Read More ā†’
Marine mechanic inspecting a boat's sterndrive serial number plate and drive shaft with a checklist before ordering a lower unit replacement

7 Checks Before Replacing an Alpha One Gen I Lower Gearcase

7 Checks Before Replacing an Alpha One Gen I Lower Gearcase Replacing an Alpha One Gen I lower gearcase can...

Read More ā†’
Flat lay of marine sterndrive installation kit including gaskets, seals, bolts, and gear lubricant needed for Alpha One lower unit installation

What Do You Need to Install an Alpha One Replacement Lower Unit?

What Do You Need to Install an Alpha One Replacement Lower Unit? A successful Alpha One lower unit installation depends...

Read More ā†’

Frequently Asked Questions

What is the difference between the SE106 and SE108 lower units?

The primary difference is rotation: the SE106 is the standard-rotation replacement, while the SE108 is the counter-rotation replacement. The SE108 manufacturer listing explicitly states that it must not replace a standard-rotation lower unit. Verify the installed drive and rotation before selecting either product.

Is an Alpha One Gen I lower unit the same as a complete sterndrive?

No. An Alpha One Gen I lower unit is only the lower gearcase portion of the sterndrive. A complete sterndrive generally combines the upper and lower assemblies, while the transom assembly, propeller, hardware, and fluids may remain separate.

Does the SE106 replace MerCruiser R, MR, and Alpha One lower units?

SEI lists the SE106 for replacement coverage across supported R, MR, and Alpha One lower-unit applications. The listing also specifies a 13-spline driveshaft and 13ā€“21 tooth count. Confirm the exact installed drive rather than ordering from model year alone.

Can the SE108 replace a standard-rotation Alpha One lower unit?

No. The SE108 is a counter-rotation replacement and must not be used for a standard-rotation application. Similar housing appearance does not make standard and counter-rotation lower units interchangeable.

How do I identify standard versus counter rotation?

Check the installed drive identifiers and propeller-blade direction instead of relying on boat position alone. Twin-engine installations may use opposite propeller directions, but converted or unclear configurations should be inspected by a qualified marine technician.

Will an Alpha One Gen I lower unit fit an Alpha One Gen II drive?

Do not assume that Alpha One Gen I and Gen II lower units are interchangeable. They belong to separate drive generations with different application requirements, so confirm the installed generation before selecting a replacement.

Can I choose an Alpha One lower unit using horsepower or boat year alone?

No. Horsepower and boat year are only supporting details. Confirm the drive family, generation, rotation, serial or part numbers, retained upper-unit compatibility and previous replacement history.

Does the SE106 work with every Alpha One gear ratio?

Do not assume universal compatibility from the SE106 model name alone. Although the listing specifies a 13ā€“21 tooth count, the retained upper assembly, existing drive configuration and complete application must still be verified.

Are these genuine Mercury MerCruiser OEM lower units?

No. The products in this collection are aftermarket SEI replacements for supported MerCruiser applications. Mercury and Alpha One references describe compatibility and cross-reference information; they do not identify the product manufacturer.

What information should I send for lower-unit fitment assistance?

Send the engine and drive serial numbers, visible part or casting numbers, confirmed rotation and clear photos of the drive and propeller. Also mention whether the installed drive has previously been replaced, converted or assembled from mixed components.

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