Sierra Water Pump Base 18-3420 for MerCruiser MR & Alpha One Drives (1985-1990), Replaces Mercury Marine 46-44292A3

The Sierra 18-3420 Water Pump Base is an aluminum housing component for the raw water pump on MerCruiser MR and Alpha One stern drives (1985-1990). The water pump base serves as the mounting structure for the pump impeller assembly, directing raw water into the cooling circuit that keeps the engine and drive running at safe operating temperatures. Because it sits directly in the raw water flow path, the base is subject to corrosion and wear over time, and a compromised base can lead to reduced water flow, overheating, or impeller seal leaks -- making this a common maintenance and repair item for boaters servicing their MerCruiser cooling system.

This water pump base is built from aluminum for durability in a marine raw-water environment and is engineered as a fit-for-fit equivalent to Mercury Marine 46-44292A3. It is intended for MR and Alpha One drive owners performing a water pump rebuild or replacing a cracked, corroded, or worn original base as part of routine cooling system maintenance.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3420
• Material: Aluminum
• Weight: 0.22 pounds
• Fitment: MerCruiser MR & Alpha One drives (1985-1990)
• UPC: 030999315922

Fitment

• MerCruiser MR & Alpha One drives (1985-1990)

Replaces (OEM)

• Mercury Marine 46-44292A3

Frequently Asked Questions

What does the Sierra 18-3420 water pump base fit? The Sierra 18-3420 is designed for MerCruiser MR and Alpha One stern drives from 1985 to 1990.

Is the Sierra 18-3420 the same as Mercury Marine 46-44292A3? Yes, the 18-3420 is a direct equivalent to Mercury Marine part number 46-44292A3 and is built to match its fit and function.

What material is the water pump base made from? This water pump base, Sierra 18-3420, is made from aluminum.

How much does the 18-3420 water pump base weigh? The Sierra 18-3420 water pump base weighs 0.22 pounds.

What role does the water pump base play in the cooling system? It is the housing that mounts the water pump impeller assembly on MerCruiser MR and Alpha One drives, directing raw water into the engine cooling circuit.

When should I replace my water pump base? A cracked, corroded, or worn water pump base like the Sierra 18-3420 should be replaced if it is causing reduced water flow, leaks, or overheating in the raw water cooling system.

Is this a genuine Mercury Marine part or an aftermarket part? The Sierra 18-3420 is an aftermarket water pump base manufactured by Sierra as an equivalent to Mercury Marine 46-44292A3, not an original Mercury Marine branded part.

Does the 18-3420 come with the impeller included? No, the Sierra 18-3420 is the water pump base housing only; it does not include the impeller.

Will the 18-3420 fit later MerCruiser Alpha One Generation II drives? The Sierra 18-3420 is specified for MR and Alpha One drives from 1985 to 1990; it is not listed for Alpha One Generation II applications.

What color or finish does the water pump base have? The Sierra 18-3420 water pump base has a natural aluminum finish.

Is Sierra 18-3420 covered by a warranty? Yes, the Sierra 18-3420 water pump base is covered under Sierra's standard limited warranty against manufacturing defects.