Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 42724A3)

Water Pump Impeller Kit – Fits Mercury R/MR/Alpha One (Replaces 42724A3)

This water pump impeller kit is designed for Mercury Marine R, MR, and Alpha One Generation I sterndrives, generally covering models from 1984 to 1990 with serial numbers ranging from 6854393 to OD469858.

It serves as a direct replacement for multiple OEM and aftermarket part numbers, ensuring a precise fit and restoring your engine's cooling performance.

• Direct Replacement: Designed to replace Mercury 42724A3, 46-37433A1, Sierra 18-3219, and GLM 12200.
• Engine Compatibility: Fits MerCruiser R, MR, and Alpha One (Generation I) sterndrive lower units.
• Core Components: The kit includes the impeller, pocket cover, and guide tube for a standard replacement.

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Mercury owners, here’s a guide to help you identify your engine’s serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Technical Specifications

Package Contents

1 x Water Pump Impeller Kit (replaces 42724A3) including:

• 1 x Pocket Cover (42724A3)
• 1 x Guide Tube (23-29804T)

Installation Guide

Professional installation is recommended. Always consult your engine's service manual for detailed instructions and torque specifications.

1. Safety First: Disconnect the battery. Ensure the engine is off and cool before beginning work.
2. Access the Lower Unit: Drain the gear lube completely and then remove the lower unit from the sterndrive assembly.
3. Disassemble Water Pump: Unscrew the bolts holding the water pump housing cover in place and remove it.
4. Remove Old Components: Carefully pull out the old impeller. Note its rotational direction. Remove the old gaskets and plates.
5. Clean All Surfaces: Thoroughly clean the housing and all mating surfaces to remove old gasket material and debris for a perfect seal.
6. Install New Kit: Lightly grease the new impeller and install it with a twisting motion in the correct rotational direction. Install the new components from the kit.
7. Reassemble Pump: Re-install the water pump housing and tighten the bolts to the manufacturer's specified torque.
8. Reinstall Lower Unit: Carefully guide the lower unit back into position and secure it.
9. Refill Lube: Fill the lower unit with the correct type and amount of fresh marine gear lube.
10. Test Operation: Attach a water source ("muffs"). Start the engine and verify a strong, consistent stream of water from the tell-tale outlet. Check for leaks.

Troubleshooting & FAQ

Q: How do I confirm this kit fits my Mercruiser sterndrive?

A: This kit is for Mercury R, MR, and Alpha One Generation I sterndrives (1984–1990). The most reliable method is to match your engine's serial number to the ranges provided in the compatibility chart (e.g., 6854393–OD469858 for Alpha One Gen I). You can also cross-reference your existing part number with those listed, such as Mercury 42724A3 or Sierra 18-3219.

Q: What are the signs my water pump impeller is failing?

A: Key symptoms include the engine overheating (especially at idle), a weak or intermittent water stream from the engine's tell-tale port, or a melted water pump housing. Upon inspection, a worn impeller may have cracked, brittle, or missing vanes.

Q: How often should I replace the raw water pump impeller?

A: As a crucial part of preventative maintenance, we recommend replacing the impeller every 1-2 years or every 100-200 hours of use. This interval should be shorter if you operate primarily in saltwater or sandy/silty conditions.

Q: Is this a complete kit?

A: This kit includes the essential impeller, pocket cover, and guide tube for a standard replacement. You will need your own tools, marine-grade grease, and potentially sealant, depending on your specific engine requirements.

Q: OEM vs. Aftermarket?

A: This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

Q: How fast do you ship and how much does it cost?

A: We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

Q: What is your warranty and return policy?

A: This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.