MG Duff 31640T4 / 31640Q4 Mercury Alpha One Gen I Standard Skeg Trim Tab Anode

MG Duff CM31640 Mercury 31640T4 / 31640Q4 Alpha One Gen I Standard Skeg Trim Tab Anode – Zinc / Magnesium

The MG Duff CM31640Z and CM31640M are standard skeg trim tab sacrificial anodes designed for compatible Mercury outboard, MerCruiser sterndrive and Honda outboard applications. Listed Mercury applications include Alpha One Generation I and Mercury 50 HP outboards.

The anode series corresponds to Mercury OEM references 31640T4 and 31640Q4 and is available in two sacrificial materials: zinc (CM31640Z) for saltwater use and magnesium (CM31640M) for freshwater use.

Both anodes measure 92 mm in outside diameter and 84 mm in height. They feature one fixing hole and a specified 7/16-14 UNC mounting thread, with a stated bolt length of 1 3/4 inches.

Technical Specifications

Available Material Variants

Both material variants share the same specified external dimensions and mounting configuration. The material and weight differ between versions.

Mercury, MerCruiser and Honda Compatibility

The MG Duff CM31640 series is listed for compatible marine propulsion applications, including:

• MerCruiser Alpha One Generation I (Alpha 1 Gen 1)
• Mercury 50 HP Outboards
• Honda Outboards requiring the corresponding standard skeg trim tab configuration

The supplied Mercury equivalent part numbers are:

• 31640T4
• 31640Q4

Specific Honda models and production years have not been provided. Confirm compatibility against the original anode, Mercury OEM reference and mounting dimensions before ordering.

Standard Skeg Trim Tab Dimensions and Mounting

The CM31640 series features a standard skeg-style trim tab configuration with the following specified measurements:

• Outside diameter: 92 mm
• Height: 84 mm
• Fixing holes: 1
• Mounting thread: 7/16-14 UNC
• Specified bolt length: 1 3/4 inch

The 7/16-14 UNC specification identifies the stated mounting thread, while the 1 3/4-inch measurement identifies the specified bolt length.

The supplied information does not confirm whether mounting hardware is included with either variant.

Zinc and Magnesium Corrosion Protection

The MG Duff CM31640 series is available in two sacrificial materials intended for different operating environments.

Zinc – CM31640Z

The zinc version is intended for saltwater applications. When correctly installed with suitable electrical continuity, the zinc corrodes preferentially, helping reduce galvanic corrosion of connected underwater metallic components.

• Material: Zinc
• Weight: 0.55 kg
• Specified Application: Saltwater

Magnesium – CM31640M

The magnesium version is intended for freshwater applications. Magnesium provides sacrificial corrosion protection in suitable freshwater systems and should not be substituted for zinc without considering the operating water conditions and equipment manufacturer's recommendations.

• Material: Magnesium
• Weight: 0.14 kg
• Specified Application: Freshwater

Inspection and Replacement

Inspect the skeg trim tab anode periodically during routine outboard or sterndrive maintenance. Replace it when the sacrificial material becomes significantly depleted, damaged or no longer securely attached.

Before fitting a replacement:

• Confirm the engine or sterndrive model.
• Check the original Mercury reference 31640T4 or 31640Q4 where applicable.
• Verify the 92 mm outside diameter and 84 mm height.
• Confirm the 7/16-14 UNC mounting thread and specified bolt length.
• Select zinc or magnesium according to the operating water conditions.
• Follow the applicable engine manufacturer's installation instructions.

Part Numbers and Cross-References

• MG Duff Zinc: CM31640Z
• MG Duff Magnesium: CM31640M
• Mercury OEM: 31640T4
• Mercury OEM: 31640Q4
• Zinc Weight: 0.55 kg
• Magnesium Weight: 0.14 kg

Scope of Supply

1 × MG Duff Mercury / MerCruiser / Honda Standard Skeg Trim Tab Anode in the selected material:

• CM31640Z – Zinc; or
• CM31640M – Magnesium.

Mounting hardware inclusion is not specified in the supplied product information.