Blog/FAQ

🔧 How to Find Your Mercruiser Engine Model Number

When replacing a Mercruiser lower unit or ordering compatible parts, you must know your Mercruiser engine model number.

Where to find it:

Stamped or printed on the engine cover

On a metal plate located on the engine block or transom bracket

Found in your owner’s manual or service records

Common Mercruiser engine model types include:

Mercruiser 3.0L

Mercruiser 4.3L

Mercruiser 5.0L

Mercruiser 5.7L

Mercruiser 6.2L

Knowing your model number ensures you purchase the correct Mercruiser compatible lower unit for your engine.

🔍 How to Locate the Serial Number on a Mercruiser Engine

Your Mercruiser serial number is the most accurate way to identify your engine and select compatible parts like replacement lower units.

Where to find the serial number:

  • On a metal tag or sticker

  • Located on the transom bracket, near the bell housing, or on the engine block

  • Format example: 0W123456

Once you find your Mercruiser serial number, you can use it to confirm compatibility for a Mercruiser Alpha One Gen II lower unit, Bravo drives, or others.

⚙️ How to Determine the Gear Ratio of a Mercruiser Lower Unit

Knowing your Mercruiser lower unit gear ratio is essential when ordering a replacement.

Common gear ratios for Mercruiser:

  • 1.47:1 – High-performance models

  • 1.62:1

  • 1.81:1

  • 1.94:1 – Often used with Mercruiser 3.0L engines

  • 2.00:1 – For heavier loads

đź›  How to check the gear ratio:

  1. Look for the stamp: On the upper housing of the lower unit (drive), you'll find a marking that starts with “R” followed by the gear ratio (e.g., R1.62, R1.94, etc.).

  2. If not visible, you can:

    • Mark the input shaft and prop shaft

    • Turn the input shaft 10 full turns

    • Count how many full turns the prop makes

    • Divide: input turns Ă· prop turns = gear ratio

Example: 10 input turns Ă· 5 prop turns = 2:1 ratio

This method helps confirm which Mercruiser replacement drive will fit your setup.

đź“Ź How to Measure Driveshaft Length on Yamaha Outboard Motors

The driveshaft length of a Yamaha outboard determines which lower unit will fit your motor. Using the wrong length can lead to performance issues or installation problems.

Standard Yamaha outboard shaft lengths:

  • Short shaft – 15 inches (38 cm)

  • Long shaft – 20 inches (51 cm)

  • Extra-long shaft – 25 inches (63 cm)

  • Ultra-long shaft – 30 inches (76 cm)

🧰 How to measure:

Measure from the mounting bracket (transom mount) down to the anti-ventilation plate (above the propeller). This is your shaft length.

For example:

  • A Yamaha 115 HP outboard is often a 20” long shaft

  • Order a Yamaha lower unit compatible with 20” shaft

đź“Ź How to Measure Driveshaft Length on Mercury Outboard Motors

Like Yamaha, your Mercury outboard driveshaft length is key to selecting the right lower unit.

Mercury shaft length standards:

  • 15” – Short shaft

  • 20” – Long shaft

  • 25” – Extra-long (XL)

  • 30” – Ultra-long (XXL)

How to measure:

Measure from the top of the transom mount to the anti-cavitation plate (flat fin above the prop).

A Mercury 150 HP EFI is usually a 20-inch long shaft, so be sure to select a Mercury compatible 20” lower unit.

🛢 What Oil Should You Use in a Boat Lower Unit?

Using the correct oil in your lower unit (gearcase) is critical for avoiding wear and failure.

Recommended oils:

âś… Mercruiser or Mercury Lower Units:

  • Mercury High Performance Gear Lube SAE 90

  • GL-5 marine-grade gear oil

âś… Yamaha Outboards:

  • Yamaha Marine Gear Oil (GL-5 SAE 90 or equivalent)

🧰 Maintenance tips:

  • Change oil every season or every 100 hours

  • Always check for water in the oil (milky color)

  • Use a lower unit gear oil pump to fill from the bottom up

Never use motor oil — only use marine-grade lower unit gear oil.